Hmmm...well, a few options here. I think a sexual interest in lesbians is a common theme among straight men, but the fact that he is consuming a lot of NON-pornographic (presuming that regular bookstore books and YouTube and tv shows are far more about lesbian relationships and life than sex alone) shows an interest in lesbians that may be unrelated to, or possibly in conjunction with a sexual appeal. It may also be that he has a crush at school who is a lesbian, or has a friend or learned of a classmate who is a lesbian and is fascinated by it. Alternatively, that he is questioning his own relationship to his sexuality and gender identity at the moment. He may be uncomfortable in his identity as a boy/man, making him feel conflicted or questioning in his attraction to women. This doesn't mean he is or isn't transgender, or gay, and don't think bringing this up to him directly is a good idea; the best thing you can do is make it clear through conversation, maybe comments upon celebrities or people you know that 1) lesbian and gay relationships are healthy, normal, loving, and ensuring they are not framed as inherently about divergent sex practices and 2) that transgender people are normal, healthy, and happy, and valid in their identities. Instilling these values in him will help to guide him away from the objectification of lesbians and lesbian relationships if it's just a sexual interest, and give him the room to explore his identity knowing his parent(s) are supportive of gay and trans identity.