Hey K., I just went through this. My son is now 3.5 and was 39 mos old when the baby was born and I could share a lot of stories. If you're interested in reading some of them send me a PM.
I will say that you're going to have to be consistent and as calm as possible.
I chose not to put diapers on my son as I felt that would only let him revert for a longer period of time. He and I cleaned up a lot of messes, but he got the picture that he was not going to wear diapers during the day (he wears one at night) that was for his baby brother.
I spend as much time as I can with him when he's not in school so that he doesn't feel left out. My house is a mess and I do a lot of housework on the weekends just so that I could split my time with both boys and not get caught up with other stuff. Soon they won't want me to play with them and I'll have enough time for chores. So, when he gets home from school, the baby usually goes down for a 2hr+ nap and that's the time we have to spend together. I usually wash bottles and if I haven't already started dinner, I do the prep work (no more than 30 min) I ask him to help and he usually does and when he doesn't I hurry up so that he and I could play. We'll go outside and play with the sandbox, or run around. I let him choose the activities, if we stay inside we usually color, draw or play make believe with his toys. His call – it’s his time and we usually do what he wants.
When he whines, I simply tell him I can't understand a word he's saying and if he continues the less I'll want to listen. We have our days and if I have to put the baby down to discipline him, he gets a time out and I remove the thing or things that was using to get in trouble. To date, I have only had to throw one toy away.
It’s going to be hard and very trying; my best advice would be to talk with him. Ask him what he would like to do and most importantly just listen. Listen to his concerns and you may be surprised at the things he’s feeling.
Congratulations on the new baby and you’ll find your comfort zone soon!
EDIT: OH, one thing I do with him is he gets a BIG Surprise at the end of the week if he shows good behavior in school too. We'll bake cookies and he could decorate them. We'll have a play date with someone. Go to Sonic and get a slushy. Something that I don't do often. At the end of the month if he's good all month at school and remains on green we'll take him to Chuck E Cheese or somewhere similar.