I agree with Zona, I have had 2 myself. First thing is to have him STOP SQUATING IMMEDIATELY.! If it is cartilage or anything else he will continue to due damage to the knee. I have less than 50% of my cartilage and I can play anything with no pain, but he will in all likely hood have to get it scoped. He will be laid up for a few days but then he can begin physical therapy and really strengthen it the right way.
I know its frustrating not being able to play but if he does this the right way now in the long run he will be much better off. He is still young and you do not want him messing with his knees. Trust me. But NO MORE LIFTING ON THAT KNEE!