Well I have a daughter who just turned 3 last month and last month I also gave birth to a baby boy... I took my daughter to all the ultra sound appts, she loved it!!! I also bought this thingie at Walmart ( I think ) that you put around your belly and you can hear the heart beat and stuff... I used that with her and she loved it being able to hear the baby. I just kept telling her every month that the baby was going to be here soon and that she was going to be a big sister and that if she wanted to help, I could find ways to include her in helping with the baby. Its hard at that age because they dont quite understand it, but keeping them up to date and showing them pictures of what the baby looks like every month is good too... I would sit down with my child and tell her, now this month our babys lungs are getting strong, or whatever. I would show her pictures of what a baby at 7 weeks looked like, then at 8 weeks, and so on... Also near the end I took her shopping, she picked out whatever she wanted to for her new baby brother. Also another good idea, you wont use for a while but what I did since my daughter WAS an only child until the baby got here, I bought her gifts before I went into have the baby but didnt give them to her unless someone who came to visit gave me a gift for me or the baby, that way you are showing your child that he is special too and that you havent forgotten him. My daughter loved it, since the baby and her both got gifts...
But back to your question, I would for sure take your son to the ultra sound appts, and keep him up to date about whats happening with the baby, that just helps them understand a little bit better.
Good luck!!!