What do I do when I cannot get my 4 yr old to stop hitting, and I have tried everything? Now, he's been suspended from childcare

My 4 yr old has been getting really bad at hitting, and I do not know what to do. I try interacting him w/children as much as possible. All he does is hit, and push constantly. I thought that the local gym's kid's club would be a great experience for him to interact w/children. Day 1- I did not get to finish my workout because I had to check him out because he punched a couple of children, then when the lady tried time-out he bit, kicked, and punched her. So we had to leave. Day two, half way through my workout I had to check him out once again because of hitting. Day 3- he pushed a child so hard it slammed the child into a wall, and left a big knot on the poor child's head..and now he is suspended from the kid's club for a week. We have tried talking, and time-outs, we do not believe in spanking. He picks on his little brother all the time as well. I fear my child is growing up to be a bully. I do not know what to do anymore. We've done the positive thing, the time-out thing, the talking to thing, now what do we do? I am soooo frustrated, and ready to rip my hair out.