Hi S.,
The number one thing NOT to do (not sure if you are or not), is to use the baby in your belly as a reason not to do things with/for your son. IE, do NOT say, 'Don't hit my belly, you're hurting the baby!' or 'I can't pick you up right now, it hurts the baby' or 'I can't play right now, the baby needs to rest.' He sounds like he may already be getting jealous of the baby without ever having seen the baby, so try to keep conversation about the baby to an absolute minimum when he is around. Make it about YOU--'Mommy is much more comfortable when you sit next to me instead of on top of me' with no mention of the baby or your belly. Secondly, buy or check out from the library every book you can find about becoming a big brother and read them to him over and over. My oldest was the same age as your son when my 2nd daughter was born, and the best book I found was one written by Mr. Rogers. I read it to her every day for those last few weeks and I could see her really thinking about all the issues she was going to have to deal with when her new little sister came home. It also gave her the positive side of being a 'big'--she could already do lots of things that her little sister wouldn't be able to do for a long time! Try and give him some sort of incentive to stay in his own room, in his own bed...is he still in a crib or has he transitioned to a 'big boy' bed? We did this for my olest a few months before her sister was born and she was very proud of the fact that she was in a 'big girl' bed. We also let her choose some new decorations for her room, the sheets and comforter, etc. Just do everything you can to make his room appealing. Also, make sure you have a nice, comforting bedtime routine that you stick to EVERY night. Try to keep in mind that, at 2 1/2, he may be having lots of really big fears and anxieties related to your pregnancy that he can't yet verbalize to you, so his actions are the way he is conveying his emotions. He needs lots and lots of reassurance. In addition, he is naturally at a stage of development in which he realizes that he is getting bigger and is capable of doing many things for himself and by himself, and that he is his own person and not just an extension of you, but that can be a really scary thing for a 2 1/2 year old to handle, so he naturally latches on to you for dear life. This would be the case whether you were expecting another baby or not--it is a normal part of development, with some children experiencing it more profoundly than others. It may just be that this developmental milestone combined with your pregnancy have totally overwhelmed him. I recommend that you take a good parenting class that will help you to cope with having 3 young children and all the challenges that will bring. Best of luck to you and your family!