The way he's treating you appears to be Narcissism. He most likely has a few to several girls who he texts and has a vague hot & cold relationship with. They are used, as are you, as acquaintances he gets attention or distraction from. Doesn't indicate NPD directly, as that is full on sociopath. From what you described it sounds more like he is not looking for Narcissistic supply as much as fighting boredom & having a kind of freedom from his gf through talking to girls like you. There is a thin border there, as far as how it can effect you though.
I am going to disagree with everybody here though. I'm probably going to get shit for this, but I think a young ISTP shows similarities with Narcissistic behavior & can definitely be confused with it. It is not about Narcissistic supply, however, as in attention for identity... so I am not saying young ISTPs are Narcissistic, by any means. I sincerely thought the ISTP I am close to had NPD. So did his family. He was extremely insecure though, and had been abused, so there were other factors with him, but I have seen much milder forms of the type of behavior yupyupyup is finding so confusing in the other ISTPs I know (when they were younger). They have similarities to NPD behavior in that they kept several acquaintances who give them freedom from close interpersonal relationships & options when it comes to activity & fighting boredom. This, plus a lack of innate empathy, can appear to be narcissistic. (Not to mention the detachment from emotions with the exception of aggressive outbursts when provoked to anger. You guys can bite like hell when you're pissed, and it often appears to be the only emotion expressed vividly.)
I'm not saying either that this guy isn't narcissistic, but they usually rush intimacy for attention over relationships with distance (such as texting), so it seems unlikely. My ISTP did something very similar to what yupyupyup is describing (though I won't go into detail because I know he'd be very unhappy with that). It wasn't narcissistic. He truly thought the girls he was texting didn't give any more care about him than he did about them.
I'd tell him you like him & put him on the spot. But don't let him mess you around if he even implies anything along the lines of liking you but not being able to leave his gf.
