H.F.
First of all, take a deep breath and relax. Having more than one child can be difficult and trying at times. BUT you have to remember, having only one can be difficult and trying at times. Relax! You will do great.
Let's look at your issues... First of all telling your little guy. Be as honest as possible without getting into unnecessary details. There are a lot of books out there for kids that address having a new baby in the family. Ask your local librarian for some suggestions. They usually have a list of ones available. We have the Berenstain Bears "Baby Makes Five". It is cute but don't stop with one. Another thing that helped for us was looking at old baby pictures of the kids with them. Remembering with them how small they were and what they could and couldn't do. For example, we might say, "Look at you in this picture! You were only a month old. You couldn't sit up by yourself, or talk, or eat anything but bottle/breastmilk. Our new baby won't be able to do much when he/she comes home either. We will have to be very careful with the new baby." Also, check at the local hospital. Ours had a class for older brothers and sisters that taught them about babies and what they could and couldn't do. It was a great class for our daughter when we were expecting our second. They went over the safety issues to make sure she knew not to pick up the baby or put a pillow or blanket over the baby's head and many other safety issues.
As for the breastfeeding... You can include this in your talks when you look at the picture. Something like, "Back then you could only drink Mommy's Milk. Do you know where Mommy's milk comes from?" And then just tell him that "when a Mommy has a baby, her breasts start to do something called lactating. All that funny word means is that they start to make milk to feed the baby. A baby doesn't have any teeth and can't sit up by themselves so they can't eat regular food like you and me. They can only have Mommy's milk and sometimes when Mommy can't be there, special milk from the store called formula that is made just for babies." Don't think that just cause you are breastfeeding you can't still cuddle your older son. When my third was born, my older son (who had just turned 3 and is a cuddler) would climb up on the chair with us or into bed with us and cuddle on the side the baby wasn't eating from. He would cuddle with me and watch his baby brother with the most loving look on his face. He still adores his baby brother a year later.
Just take a deep breath and take it all one step at a time. If I were you, I would wait until I was at least 15 weeks along before I told my little guy. This way, you will have had your sonogram and a few of the other tests and will know that everything is all right. It will also give you some time to check into books for your son. One other thing that really helped with my kids was the book "Your Pregnancy, Week by Week". We would look at the book almost daily. They loved to look at the pictures of what a growing fetus looks like and loved the idea that this was how their new baby looked each week. It also tells what the baby is doing/what is new with the baby each week. My older son would snuggle in my bed with me and we would look at the picture of what the baby would look like and how big it was and I would read what was happening with the baby that week. One thing you want to be sure to tell your son when you are talking about the new baby... When the new baby comes, he/she will sleep a lot but only a little at a time. That means that Mommy won't get much sleep and Mommy will be very tired for several weeks after the baby is born. Tell him that Mommy will really need his help then. My son helps by throwing away dirty diapers, getting clean diapers, and running any little errands or holding things for me as needed and as per his ability. It made him feel more included and less likely to act out.
Anyhow, I hope this helps. Feel free to email me if you start to panic again and need someone to ask how it worked out for them or just need someone to tell you to calm down and take a deep breath because it will all be okay. ;)