The dad has some problems, too. One is emotional maturity and another is that he has his kid in public school. If he actually cared about the boy he would home school him.
His options as far as controlling other parents kids especially preteen ones would be very limited. Educating parents is a logical response. The parents of the kids doing this likely have no idea the kid exist. His response is putting out some public shaming sorely lacking these days. Age of the offending kids makes a difference also. When I was about 12 a 16 year old hit me I had walked 2 miles to a store to wait for my dad to drive by on his way home from work for a ride back home. He saw I was upset and I told him. The 16 year old was in a car in the drivers seat. Thus legally assumed to be an adult. He asked why did you hit him? The 16 year old said because I don't like him. Dad said I don't like you either and punched him. The 16 yr olds dad was a passenger and had Polio. The guy never hit me again.
Men crying is not a show of weakness. The shortest verse in The Bible is "Jesus Wept". I saw my dad do it more times than I can remember when life events came crashing down on him and that man carried a heavy load. I'm proud to be his son who also has shed countless tears when sometimes life sends you to your knees in a breaking point. Dad also had to get me back in school after three suspensions for fighting. I'd get hit, I'd hit back, I got three days suspension. That was in high school and after I had the ability to swing and connect a punch.
I made another post before this one as to what I think is the best solution and it comes from experience.