In my opinion and from own experience it is wrong to force someone to stop something. I would recommed you to talk to him and figure out why he is wearing your clothes. But do this in privat and stay calm, he is your son and you are his mother, the only woman so far he can really trust (you don't want to lose this trust). Maybe he is not sure about his sexual orientation yet, I mean he's only 14, it can be really confusing and maybe it is an issue which is bothering him and he has no one to talk to, you should take the first step. I can totally understand that this is also confusing you but only talking with him will make things clear for everyone. And don't go to a therapist, this only leads to him feeling wrong and not accepted and things will go only worse from that point. Be the better person.