Astris, watch your six. I don't care if he's an INTP, THAT is simply unacceptable, especially with the roller coaster he's put you on. I would be pitching an Fe-fit right now if I had an INTP girlfriend who was putting me through that (though that is EXTREMELY immature behavior for an INTP on average, as an adult). I swear, it looks like he's about to get laid for two weeks...
I'm a man. I'm a loyal man, and I will be a monogomous man when the time comes... However, from a man's mind, and a man's sexual perspective, and a societal perspective, he's <perhaps subconciously> looking to get laid. I'm sorry.
He may not know what he's thinking. Men often don't focus on our own motivations for doing things. I will tell you that it's likely, in his head, that he's told himself it's okay, that he's committed to you, and that she and he aren't...BUT, again, he might not be aware of his subconcious attraction to the other girl he's about to give one-on-one precious time with.
If I were you (and I sort of am in a way), I would insist on inviting myself on that trip. This is a major violation...even he should see it. If you don't go, I would end the relationship. I don't care about how much he takes his attention away, and sulks, if his intentions toward you are as they appear, he shouldn't take issue missing one camping trip alone with a girl...in theory...he's completely plutonic over (and therefore shouldn't desire to spend a significant amount of time with her alone). Things just don't add up.