Cats are bullies and if this brute has not been castrated he will be a big bruiser. Some people with male animals fail to understand that although their cat won't get pregnant they will father kittens, fight, bully, fight and then impregnate another poor lady cat.
If he has owners try and approach them. The water method of deterring it is okay when you are around but your cat might want to go out to play with his pals without taking his mum along too.
The good news is that if you boy is getting wounded in his front areas it means that he is fighting back. If all the wounds were on his bum area it would mean that he was running for cover.
Try these sites to see if there is any help for your lad -
http://icatcare.org/advice
http://www.cats.org.uk/
My boy cat gets beaten up by his sister - less so now that they are getting older. We were forever at the vet, but he has learned how to out-run her.