Hitting

My sons a hitter. I only found out a week ago. A kid this morning was discussing about him to a relief teacher and the kids were like oh he hits everyone he's not here today. When one girl saw he was she was like oh no. I've already spoken to the teachers and they said its common and others do it aswell but all I here is kids talking about my son. He will be going to primary with most of these kids next year so I'm worried even if he stops they won't want to play with him anymore. I can imagine that they will start to exclude him. He has speech delay so the teachers say it's a lot of frustration behind his hitting. He says he has two friends and pretends to talk to them on a pretend phone but I think he's starting to become lonely and now has an imaginary friend in the back yard. How can I help him bond friendships and stop hitting the other children?