D.T.
Wow aren't kids fun!:) I have three little boys. 5yrs-3yrs-10 1/2mo. When the baby was born my now three year old was a lot the same. It was hard and I found that if I took time with him and the baby. Letting the baby see him and try to watch him as well, he started doing a lot better. I know this may sound crazy, but it is more controlled and if you sit with him. Also try and let him help with things (if you don't already), let him help get the baby wipes or put the diapers in the garbage. With my now 3yr old he just wanted to be part of the babies life to, but he was being aggressive because he didn't get to help.
I hope this helps or at least gives some new ideas.
Good Luck! It will end sooner than you think!