I would guess that maybe yes, he has freaked out a bit - travelling together is very intense as you'll be together 24 hours a day, which you aren't at home, even when living together. Although you might both be working while you travel, it's still more intense, as there's a lot more you have to rely on each other for when you're away; although you'll meet other travellers, all your main support systems which you have at home (family and friends) won't be around. He's a guy, so this kind of thing, however much he loves you, will probably be scaring him
I think it's unlikely he's been seeing someone else -(though obviously I don't know that for sure). I would imagine that you would have sensed that particular kind of distance from him, way before he said anything about wanting to finish.
He sounds confused: he wants to finish, but then again still wants to travel with you (and I don't think he's saying that because he feels bad about letting you down, if he didn't want to, he wouldn't). And that he loves you and you're the best thing that's happened to him. I don't know, maybe he felt more of that pressure because he's a twin, and it was his twin who was getting married, so it was a double whammy, combined with the comments about when the two of you were going to get hitched.
To be honest, I would talk to him again. Don't ask him what he wants, ask him what he feels and tell HIM how YOU feel - both to try and clarify what it is he's actually feeling, and also to make sure that whatever you end up doing re travelling, it's not just falling in with the way he wants it - ie travelling with you till he decides, then separating. And travelling on what basis together? as friends, or more? You need to feel happy enough with whatever is decided between you, however much you might feel now that if you don't travel with him, it'll mean being on your own, or if you don't agree with the way he wants things, you'll lose him altogether etc etc - I don't know what's going on in your mind, just guessing here. You don't need to say it in an accusing way, but after so long, and with these plans you've had, you deserve to know more - ok, he's a man and will find it difficult to know what he feels let alone say it, but just saying he's freaked out isn't really enough. Maybe you could ask him how he visualised your time away together previously. It's possible maybe all he needs is the chance to be a warrior man and go off into the jungle by himself (symbolically speaking) for times while you're away - maybe he'd be happy with a situation that you're basically travelling together (and together as boyfriend/girlfriend) but that there might be times when he (or YOU!) want to go off for a month or a couple of weeks here and there, just to experience the lone travelling thing. Maybe he felt he couldn't say that to you all the time you were planning it, so now he has freaked out completely and thinks he has to be really extreme in order to feel he has space. Honestly, it really annoys me how men always say women are complicated - trying to find your way round their emotional life is harder than finding your way round the world.
Hell, I don't know. I could go on forever. I think it's important you work out what it is you're really feeling about everything. Sounds like a really painful place to be in, but I'd take heart from his confusion (if you can) - if it was clear cut he didn't want to be with you, then he would cancel the travelling altogether (or at least cancel going with you). Good luck