Whether you're just friends or in a relationship, distance is super tough on anyone. As time goes by, the person that moved begins to build a new group of friends, and it can make things awkward. In this case, it sounds like the actual nature of the relationship changed, and you two aren't exactly on the same page.
The fact that he's become super flirty is fine, but the entire picture makes it sound like he's just fishing for some sexting and nudes. That distance between you two provides a bit of a safety blanket, as there's very little chance of seeing one another in person on a regular basis. Due to that, he's feeling a bit more bold and trying to see where things might lead.
Sadly, I don't think he's looking for anything serious at all, and his behavior when he comes into town completely supports that theory. If he really wanted more than just some online action, he'd make you a priority and have some physical encounters to go along with the phone stuff. He's not being shy, he just wants a very unique relationship with you.
I don't think this is necessarily a bad thing, but the two of you need to have a conversation to set expectations if it's something you're into. If you're okay with just flirting and maybe more when he's at his new home, and being just friends when he's in town, then there's no reason not to go for it. In some cases, this ends up turning into a relationship; but most of the time, it's just sexting with someone you already know.
One bit of warning: once you cross the line of nudes, you can't ever go back to the relationship you had previously. It's the idea of things that can't be unseen. So if you're down to go that route, make sure you think it through a few times before you act on it. If you're not into it, know where your line is and stick to it.
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