My 9yo seriously injured someone at the park.... what can I do?

The other night my 9yo came home from park with blood on his coat, asked him what happened and he said he was with mate when some other boys were hassling them, and son said he smashed their face with a brick. I asked him about it and tried to get as much info why... these lads have been bothering them for a while and think its same people who sometimes try egging our windows. Theyve been doing this on and off for the last year, it started in school but the school washed their hands of it because the "behaviour in school premises has improved" but all they do is just hassle my kid outside of school.

I spoke with his mates dad and they confirmed for me what lad said, said some other boys fwho dont go to their school were hassling them and things got physical, they pushed back and my son picked up a big lump of stone and bashed one of the kids in the face, mightve broken his teeth.

My son has been out of school because he was all worked up about it... I have had him spending the day with my sister in law while we sort things out. I am really panicking here because I dont know if this means some authority will try getting involved and dictating how I deal with my kids, what my son does etc and how we raise him

He is only 9, his birthday is a month of... I know the law means they can arrest kids over 10 but I really dont know much past that when I try looking it up. This group of lads have been hassle before trying to start up trouble, some of them are older too and should know better. I just dont see myself allowing my son to be taken away for standing up when being pushed about.

What should I prepare for with my son? I dont want social services involved, my wife has had a bad experience with them as a kid herself, and I dont trust authorities to be sensible here... I get he hurt this kid but theyve been hurting him before, and he just thinks he was just getting on top of it by hurting them more so theyd stop.

Ive had people suggest I should punish my son, isolate him, dont let him go to the park... I dont see how thats a solution, hes a kid and should be able to go to the park. if people didnt hassle him he wouldnt have retaliated