Mastubation in teens is becoming normal as most are still figuring out their body, hormones & puberty.
I strongly believe you tell his Father,he will be in good position to handle it & guide his Son better.
DON'T RESPECT HIS PRIVACY ANYTHING.
My 16- year-old nephew is m@sturb@ting, and I'm in a fix about whether I should tell his parents.
I've noticed this for three weeks now because he stays longer in the bedroom. He can spend about an hour there. The recent incident was bad: he left the bathroom hurriedly, then immediately left the bedroom. I entered and saw a drop of sperm inside the closet. It didn't flush very well.
I feel bad because he is slow and forgets things easily. I don't know if it's my business to tell them. I'm in a fix, honestly.