Don't read too much into what type of pron your bf is looking at. Maybe he's just curious or whatever. All forms of sexual preferences are available on the net and many check out types of pron they wouldn't prefer irl. You're both still very young, so both of you still have a lot to explore when it comes to sex.

As for the 'teen' part: that's very probably totally harmless. It's a category of young men/women, but all are (as long as it's a legitimate website) above the legal age, so no minors there.

As for advise, if you don't talk about this, it's going to leave a shadow over your relationship, as it's obviously bothering you deeply. So, the only thing to do is to be open about it and talk. Try not to be judgmental, but do state your concerns. After that, you can make up your mind on whether there is a future for the both of you or not.