A friend of mine had the same problem with her 3 year old when the new baby came along. It was so bad infact that the 3 year old regressed and wanted daipers and bottles again, as well as to be carried around all the time.
We discussed the problem, and I came up with an idea which seems to have helped. I suggested she get her daugher a couple of goldfish which her she could look after herself, and it might make her feel better and take some of the attention away from the new baby. It seems to have worked so far, and things are looking good again.
I can only suggest the same thing. Goldfish aren't expensive, and it might be an idea for your son to choose a couple himself, and to choose the tank too You can get some good plastic tanks in the pet stores. He would enjoy feeding them and cleaning them out, but I would suggest that you keep them out of reach at all other times.
I hope this helps.