Try setting aside some time each day for your son to get your undivided attention, and some time for him to get attention from his dad as well. If you can pinpoint the time of day when he has been having problems, spend some time with him just before that. Is it late afternoon when everyone is tired? If so, go for a walk or play a game with your son before that time and then make sure he gets some time to rest.
Check with your public library or a local bookstore for the many books you can share with your son on having a new baby in the family -- Peter's Chair is a classic. Pick a time when your son is calm and read them with him. You may want to continue to do this several times a week for the next few months.
If the problems continue, talk with your pediatrician about a therapist in your community who can help your son.