15 yo son masturbates a lot and then gets sad and tells me. Told him it's ok but he still gets down. Think he also likes a boy. Should I ask him?

Friendly chat. Ask him why he feels sad after masturbating? Perhaps he is getting input from someone else who tells him it's a bad thing. Ask him if he has any worries he'd like to discuss with you. Be open, supportive. Perhaps he'd feel more comfortable discussing the issue with a counsellor. Just let him know you're there for him whenever he wishes to talk.
Be supportive. Direct confrontation will worsen anxiety, enlarge social gap, and may cause acting out behavior. Be clear you are available for support, questions, unconditional love as parent, and willing to help. Can ask about significant others, and include same sex as "on table, " so he feels less anxious/ashamed and more willing to talk. May eventually need professional help to navigate conversation.