Interesting answers, since I was the exact opposite. I know this is gonna be long (too long, probably) but I'm just laying out what we did to prepare my oldest for our awesome baby. It was all over the space of 8-9 months. My son was 2 most of the pregnancy; he turned 3 one month before the baby was born. We waited til like 2 months, when it was "typically safe" to know the baby "took" and wouldn't be a miscarriage.
Once that was done, we just came to him very happy and excited and said "Do you know how your good friend __ is a big brother to __? (his playmate had a big brother of 6 that my son adored)....GUESS WHAT!!! You're going to be a bubba just like him! YOU are going to be a big brother!" My son's mouth dropped open and he gasped, trying to take in that he would be like "Bubba", the boy he thought was so cool. Young kids will quickly read your emotions and go with them, so don't be all trepidatious when telling him, be excited and happy.
We would talk about the baby coming (it's gonna be awhile, but we'll wait for it), we talked about what big brothers do, etc. DO NOT make the mistake of asking if he wants a brother or sister, since he doesn't have a choice. We were having a baby, until we knew it was a boy and then we told him "Oh my goodness, it's going to be a baby BOY!" We went to the library and got nearly every book they had over the course of the pregnancy on bringing home a baby, being a big brother, life when the new baby comes, etc. We signed him up for a sibling class for young children at our hospital that I attended with him which was very very cool.,
He wore his big brother Tshirt very proudly and would tell everyone that made eye contact with him "I a big brother!" or "mommys baby in tummy". He did get upset when he thought the baby was kicking me and being mean (he's always been very protective), so I told him the baby could hear him and was excited to meet him; he was going "knock knock, lemme out!"....that made my son laugh, and he liked that answer.
We gave our son a "big brother room" because he's going to be so big now, with a fresh coat of paint and a few new things to spiff up his room a bit. We cleaned out his room as well, and took care of his room before we did decorating and setting up for the baby. That way he wasn't forgotten or all the focus on the baby (but it worked well because we didn't know for sure whether we were having a boy or girl until later). He went to every doctor visit with us! He got to hear the baby's heartbeat, sometimes he got to see the baby although he was excited to do it, but wasn't impressed with the "pictures" on the TV. He even went with us to see the 3D sonograms which were awesome. He seemed intrigued when the video was clear, but would get a little clingy and jealous of daddy during this time. Still, he kept wanting to go with us and it is a memory and experience he has had and can feel big about. We put him in a 2 day/week mother's day out program that helped ME rest 10 hours a week, gave him a social life with some friends, and he is big because we call it "school" not "mother's day out". We did that as soon as we could (but it was the beginning of the third trimester before we could) so that he would have a place to go when I needed some quiet time with the baby later, but he wouldn't get the idea that he was sent off because of the baby, ya know? I also got him a new Spiderman backpack that I knew he'd think was awesome, and made a "big brother bag" for the hospital with a couple new hot wheels, a coloring book and new crayons, a comic book and a big brother book, a couple snacks, and a disposable camera that he could use to take photos of whatever he thought interesting. All stuff he'd never seen so it was new, special, and kept him quietly entertained while hanging out after the baby was born. He didn't see this bag until we were having the baby.
We made arrangements for my mother to come, which was awesome because he rarely gets to see her. The deal was that she would come and keep my son with her in our house while I was in the hospital the 2-3 days after the baby was born. I left a folder on the table with our basic daily/weekly routine, a page with important phone numbers/addresses (doctor, hospital, our cell phones, his school, the emergency lady he may be staying with when she came to town if we were already in the hospital, and 2 neighbors / friends if she needed assistance with anything). I chose a friend from Bible Study to keep him if I went into labor before my mom could get to us (but had a couple backups in mind, and they said yes), and I invited her to our house for dinner, then we went to her house for dinner so he could play with her dogs and explore her home and feel comfortable. She was his "friend" and he loves her, so that was awesome. He felt it was just fine by the time he actually went there. He helped me prepare a month's worth of meals in advance (with a new baby, guests that don't cook, etc I knew I wouldn't want to be worried about that) and helped me stack it all in the deep freezer. I'd chop stuff up and he'd dump things into bowls and stir, little things like that---gave me special time with him, and he was helping me get ready for his little brother. That made him feel important.
We found a book that had OUR plan in it: a big brother's grandma comes to visit him and play while mommy is in the hospital, they get a phone call, and find out he's a big brother. The baby comes home in a few days, and doesn't do much but it's special to be a big brother anyway. We read that book together daily (mostly he "read" it to me after the first few days) for weeks. My hospital bag was already packed and in the car, as was the baby carseat, weeks before the baby came. When the time came, we just said "Guess what! It's time.....I'm going to go to the hospital so the doctor can help us get the baby out and you're gonna go play with __ and her dogs!" He thought the sleepover was for him, lol, so he was excited about that. My friend watched him while mom was at the hospital, but after the first few hours she went home to be grandma.
He was very happy about his own busy life, but always knew what was going on, and I wouldn't have it any other way. For one thing, it was an experience for him too, and I wouldn't want to take that away from him. Secondly, preparation is a GOOD thing....mentally, emotionally, physically.....it's something to feel the downtime, and helps SO much when the downtime is over. And what's wrong with questions? I feel that if you talk openly to your children (but on age appropriate levels) they don't ask as many questions, or at least I don't notice it because it's a conversation not being assaulted with questions.
Even with all the preparation and excitement, he is still just a 3 year old and has always been a daddy's boy. He was very happy and proud, held the baby at the hospital, etc but we did go through a few "quiet days" where he wouldn't look at the baby up close when we came home. My parents were here a week and he'd spend time back and forth between us and them, but we just made sure to not push the baby on him ("Do you want to hold him? No? Ok--when you feel ready"). We did quiet things together like all pile up on the couch together (sleeping baby included) to watch cartoons or a movie, and that gave reassurance and understanding that we are ALL together. On the 2nd day, he crept over and touched his toes. I put his feet up against the babie's feet, hands against hands, etc and showed how he's just a little boy, just like big brother, but he's very little right now so we have to be very "soft" with him. He seemed intrigued by that. The 3rd day, he gave the baby a paci. By the end of the week he'd be more engaging and ok. No matter how much preparation you do, it's still going to be a shock to go from the theory of a baby to the actual "this kid isn't leaving?" feelings. We don't push. But now he will hold the baby with us (not often though-he IS very heavy for a baby), he plays on the floor with a rattle or whatever, sings to him, they chat. He explains that the baby can't play soccer because he has little feet, and I agree. But I showed him his own baby pictures and how he grew up to ride horses and play soccer, and his little brother will grow and do fun things too. I think the positive anticipation helped the transitional time go by quickly and without tears. Good luck, congratulations, and think about letting your son be part of this special time!