How do we cut off a damaging friendship with the boy next door.

We have a nine year old son who's best friend is a boy across the street who is sometimes a great and adoring friend and sometimes a psychopath. ( damaging property, lashing out, throwing balls at people's faces in anger, lack of emotion and empathy as a few examples) We'd cut their relationship off before for several weeks because together they had hurt another friend, not terribly, but in fits of anger, so thought they needed a breather. About a month later the boys really missed each other and promised to make good choices. And we watched them closely. Everything seemed fine. Then last night I found a bruise on my sons chest right above his nipple. I remembered that the neighbor boys Dad pinched his teenage boy in this same location when he was penalizing him, awful right? Put 2 and 2 together and asked my son if the teenage boy had done this to him, and he admitted he had while they were wrestling. (Alarms going off!) Our son talked to my husband about it at length and the bruise was just the beginning. The wrestling the boys (my son, his friend and sometime the 14 year old) were doing was alarming--way too much. We thing wrestling is good for boys, but the brothers were using humiliating tactics. So, that relationship with the brothers is over for good. Now we need ideas or to hear about similar experiences. We're going to talk to the parents of course but beyond that, these are our neighbors and the boys are often outside and playing everywhere. We share the neighborhood pool and we share friends and the boys ride the bus together and are also in scouts together. It's like we're glued together and I just want to pack up move away. Help! I need some ideas to keep the peace and keep and help my son move on. The outdoor play time he shared with his friend was a big part of his life, how can we replace that? via /r/Parenting http://ift.tt/2olK0vm