Tell him masturbation is normal and that almost every man past puberty does it. Tell him that it is an activity to do only in the privacy of his own room when he will not be interrupted, and that it is only considered a problem or abnormal if it becomes so obsessive that he does it in preference to normal life activities like sports or school or work or friends. For your own education, google "masturbation myths" so you know for yourself that what he is doing will not harm him, stunt his growth, cause him to become sterile, grow hair on the palms of his hands, or anything else.
You're overreacting. Masturbation is good for everyone, and it isn't a sin either. Don't make your kid ashamed of this. It doesn't matter though, because there's really no way you can stop him. As long as he's not doing it too much or too little, then he's fine. Masturbation releases tension, and calms people down. I'm sure you've masturbated too, anyway. Come on, it's not bad at all. At least he's masturbating instead of having sex.
Most boys who haven't started puberty masterbate. It isn't a bad thing,I'm a guy I should know! It is actually a really good stress reliever and some teens do become obsessed witrh it.but don't worry it will eventually get old. Just tell him its ok and that its normal.
does matsurbation cause growth to stop or anything?