I saw earlier on this thread that you should tell him it is a mortal sin and urge him to go to confession. This is solid advice, but to elaborate on it more you should ask him if he knew it was sin first and if he did then that would be a time for corporal punishment. Or better yet have the father do it so your son will feel more comfortable discussing it. The key here is to not discourage your child coming to you with questions because they encourage masturbation in schools whether they be Catholic, private or public, so there is a very good chance that he did not know and especially since he is 13 because that is around the time when they start their sexual education programs.
But if you punish ignorance you'll merely alienate your child and encourage him to do it more frequently, try and remember education is the priority.