All these people saying let him stand up for himself and be brave are idiots. I know a kid who I grew up with, went through the same thing your kid did at the same age, cept there was a gang of em giving it to this boy. Eventually he did "stand up for himself and be brave" after a couple of years. And he got expelled from school for it. Now has terrible anger problems, has a criminal record, was a heavy drug abuser for several years, can't make proper relationships, can't hold down jobs and to be honest, if I hadn't been friends with him for such a long time, I probably wouldn't like him at all! Very few other people do.
Best thing to do if this is happening to your kid is, MOVE YOUR KID SOMEWHERE ELSE. And enrol him in a martial arts school to give him his confidence back. If it was me, I would teach him how to get his own back on the bully with a couple of moves but if you don't know how... it's not -as- important as getting the kid out of there. ASAP. They are old enough to understand what being pissed on is all about. Both the one doing it and the one getting pissed on. This kind of $#%^ ruins children.
Trust me.
If you ever come back let me know how your son is getting on.