Thank you for replying, everyone. It really means a lot to me because I just really feel like I need to talk to someone about this issue.

Miss Laura, I honestly do not know to if he's gay. Like I said, he claims that he is, but it was very sudden, and the way he phrase it, were just really awkward. I only know that he used to watch heterosexual porn sometimes but according to his inter history now, he stopped doing that. In fact, he stopped watching porn altogether, now he only reads rape/abuse stories.
His relationship with father was never really perfect. Our father was always very strict with him because he had quite high expectations of him, especially when it comes to my brother's piano lessons. They rarely argued, however, my brother often tried to avoid spending time with father. I can't remember him ever being affectionate with father like he was with mother, they were quite distanced I guess.

I'm not sure if threatening to kick him out is a good idea, I'm worried that it may make him even worse, I guess my house seems like a comfort zone to him? Because he likes to spend time here. I'm not sure if college notified our parents. If it did, they didn't say anything. Still, on the other hand, he's 17 so maybe college isn't really that bothered with attendance anymore.

also_depr, him being depressed isn't really my biggest concern, I understand that everyone has their ups and downs, I'm more concerned about the other issues, like his sexual behavior and unsupervised medication use. But I will try to make him trust me more, just like you said.

Hatter08, I see, I guess I'm a bit uneducated in that aspect, I will try to do some more research.

Harley47, I honestly don't think that father could do anything of that sort to him. He might've been strict when it comes to my brother but I can't see him as an abuser. Anyway, he never even tried to harm me in such a way.

I understand.. So it won't help if I try to force him into therapy... You know, when I try to talk to him, he insists that he's fine, even though he obviously isn't. I don't really know what to say to him anymore.

Some of you suggested that i should directly confront him about the possibility of abuse but I have no idea how to do this. I don't want to be insensitive about this, is it alright if I ask something like "Did someone hurt you in anyway? Is that what makes you so depressed?" Or do you think that's going to just make him even more secretive about everything? Or maybe I should try something less direct? For example watch some sort of film with abuse, just to test his reaction? Or is it a bad idea? I'm really not good at this... I feel guilty because he's under my care now and I just totally don't know how to act around him.

Again, thanks for all your advice everyone, I really appreciate it.