If his spontaneous exuberance lacks discrimination, train him to properly channel it, but do not turn him into a girl. Invite other boys over and go out to watch them wrestle. I always rooted for the one on the bottom, whether it was my boy or another. When other boys visited, our boys would yell and run out of the house to get something exciting going. I expected them to climb, jump, wrestle, push, fall, get hurt, and get up to try it all again.
Children establish their own social rules.
Your son would have been better left alone. He admitted attacking the bigger fellow. A little thrashing from the big boys would teach him the limits. I would only intervene if I felt that the fight was going to cause serious damage to one of them. Teach your boys to be kind and loving toward their neighbor.