I caught my 13 year old son masturbating?

He doesn't know I caught him. He carelessly left his door slightly open and to my horror I accidently caught a glimpse of what he was doing. So, I'm thinking, what am supposed to do, barge in there and punish him? I didn't want to embarrass him however. I thought maybe I could watch him but that would be rude. Should I talk to him about it, tell the wife, or just let it go?