No. don’t go to the police. yet. Get the names and numbers of all the witnesses in this case. Tell the “Talmid Chocham/abuser” that if he ever lays a hand on your son again you will be forced to go to the police. This time is a warning only, as you understand that he lost it. But if it happens again you will be forced to take action.
If you go to the police, the police will come to him and he will say that he acted in self defense, the boy tripped, etc etc. and make up all sorts of lies to avoid getting in trouble.
He will be upset and give your son looks in shul and your son will be afraid to cross the street. He, his wife, and children will tell everyone how awful you guys are. You have a son who throws rocks on shabbos, an older one who isn’t even frum, and you had the nerve to call police on them. You will be forced to leave your shul or chas v’shalom move!
No, I say don’t go to the police at this point. Don’t make a big machlokes right now. You want to remain in your house, your neighborhood, and for your children to live without fear of these bullies. If it happens another time, then you have evidence that it’s the second time. I would even hire a lawyer and present the so called talmid chocham a letter from the lawyer with names of witnesses with a legal letterhead, etc. but I don’t know your finances.
As for your son, I think you should give him karate lessons. And if the guy EVER starts with him again he will regret it. (I don’t know if there is a frum group for karate lessons or how much it is, but in america many frum kids do it).