Thank you all so much. The guy he's thinking of seeing is perfectly nice, really, he's just much better friends with my boyfriend then he is with me. I don't doubt my boyfriend's estimation of him, I'm just afraid that if it turns out we can't really be friends, it will strain the relationship too much. I really want to be is friend, though. I'm hesitant to get to know him more because he's got a lot on his plate right now--getting out from under an abusive parent--and I don't know if trying to talk to him would put pressure on him to socialize when he doesn't feel like it. He and my boyfriend live in the same city, so I guess some of my insecurities come from the fact that they can so easily spend time together. I would honestly be really happy if he could be around someone when he needs to be. My boyfriend and I are both moving soon to our respective colleges, where we'll be about six hours away from each other and able to visit.
To answer your questions, Magdlyn: He's known he's poly pretty much all his life--he says when he was younger his ideal relationship involved three people and he didn't know for a long time that people consider that relationship "wrong." I don't know what experience he's had with relationships before, though. The new love interest, like I said, lives in the same city as he does, but very soon my boyfriend will be moving to college. They already spend a lot of time together (and I don't grudge them that; I'm really happy my boyfriend has someone to spend time with in person).
As for dealing with jealousy and envy--I don't know. Before, when I'd bottle it, after a few hours I'd be curled up on my bed and wallowing in sadness, so I obviously need to do something. The poly resources I've read just suggest having something to do. Soon I'll have schoolwork to do, and in the meantime, I can do what I usually do when I have feelings I don't know what to do with; write it out. That always helps me.
There's nothing wrong with this guy, we just don't mesh as well as he does with my boyfriend, but I would never disrespect him or try to turn my boyfriend against him.
I think the next step is to work on getting to know him more, and tell my boyfriend I'm cool with them being together while talking through it.
(And I was a little shy to say again that it's a long distance relationship because I was afraid of not being taken seriously but I'm glad that's not the case.)