3 year old and still hitting?

my son is almost three hes always had a tendancy to hit it started at 11 months when he became good at walking i decidd to start taking him to play groups but i was always and forever pulling him off another child it was very distressing but i never gave up taking him places it has got so much better as times past but has not stopped his pre school teacher says he gets frustrated cause he cant express himself he didnt start talking till 2 and its been a little slow hes very little consentraction so its hard to teach him i seem to put so much time in and see little result when ever he wants to play with a child hel go up to them and just push them now or hold there arm or hand and wont let go or just swing his arms around and throw things what can i do i know hes a boy but none of his friends are the same way