hi! not knowing the whole situation, this is what i think. he may be jealous of the other children coming into his home and playing with his mommy. also, make sure that when he hits, not to spank or slap his hand as dicipline. that only says..."hey, you can't hit, but mommy can" which would be confusing. try to catch him when he is being nice to children, and reward him or make special mention when he is nice to them, to make him realize that this is good behavior, and he recieves praise for behavior like that.
hope this helps a bit, good luck!

I agree that spanking is not the answer! I am not a spanker or a hand slapper at all.
I don't know if he is jealous. As I said, he doesn't do it out of anger...he laughs the whole time. I think if he was jealous, he would try to hit whoever I am playing with at the time, and he doesn't do that. I've tried distracting him, removing him, and telling him we do not hit people. Unfortunately (or fortunately, depending on how you look at it), he is being as persistant as I am.
He is my 3rd child and totally is the boss right now! LOL I never went through this phase with my older children.
STILL LOOKING FOR ADVICE!!!!