Quite a turn of events, reverses the roles of your yf and his gf in how you thought things were happening.

This is a huge issue and not something I can give great (I think) advice. You cannot do absolutely nothing but you have to know that you did not cause this. Teenage boys are not the same as adults but they are mature enough to understand that what your yf has done is wrong (not all believe it is wrong but are capable of understanding it). They are also capable of controlling themselves well enough not to do what he has done.

You giving him condoms does not make you culpable of his action. Giving them condoms means a 13yr old and 15yr old are not now parents. With the way he is acting condoms or no he would have done it anyway

He pressured his girlfriend who is 2yrs younger than him into sex she did not want, so rape.

It is good you stood up for your yf's girlfriend. Next time you see him you will have to do the hard thing and bring up this subject, don't let it be avoided. Make sure he knows he could get in serious trouble for what he did, that he is fully culpable for hos action, he is 15, not 5. He's had a hard time growing up but that is no excuse for a person doing immoral things.

He HAS to immediately start changing his life and outlook and actions or things are going to get real bad for him real fast. This is simply how it works. Some people get away with doing shit like this for awhile but eventually it will catch up with him. He may get an STD, go to prison, get seriously hurt somehow, I don't know but living your life like that will result in something bad. Some people have to learn things the hard way, but it is so hard watching these people fall knowing they could stop if only they wanted to. And I guarantee if he does not make major changes in his life he is going to fall and hard.

He needs to recognize what he has done to so many people he seems to care about. He needs to evaluate how he really feels about these people. Realize how much pain he is causing for others. He needs to apologize for real to all people involved.

Many people have problems with empathy and that seems to be one of his issues, that and a wanting to fulfill all his hedonistic desires. Combine those two things in someone with a high sex drive and you get this.

It took me a little while after I saw this post to respond cause I was not sure what to say. Maybe I'll think of more later. I hope this helps. Good luck Mason.