i have some experience with istp men. my hubby is one. i just was in a weird, sort-of intimate relationship with one, and i've crushed on others.
i think istps come in two flavors: self-centered and other-centered. the former can possibly grow out of it or not, but in the meantime, you're pretty screwed as his motivation will be totally about and for HIM; the latter is pretty good for a relationship if you can find a way to mesh with him and the timing is right and all that other stuff.
i think (based on numerous convos with my husband--haha i can put one in the torture chair and force him to divulge his secrets!! buahahahahah) your guy found you to be a challenge from the time you turned the initial romantic feelings into a friendship. from the beginning. i think his constant texts and communications were to get you. i know it sounds completely unbelievable and manipulative, but young, or self-centered or *insert adjective* istps have been known to do worse for a piece of ass. it might not have been completely done consciously either.
of course he LIKED/S you or he never would have invested that kind of time in you. now he's ready to move on. istps live in the present moment. he is not going to get much out of a ltr or friendship with you. what? talk about feelings? haha. talk about school? talk? not likely, unless you are one of his guy friends and you're getting drunk with him.
then once he got to you and you *made out* (i'm assuming, not to be too vulgar, that he got his rox off

) he realized the challenge was gone and he was off to school soon anyway, so ce la vi. plus he's already dealing with a gf and heading off to college.......which is a lot to deal with in itself for an immature istp.
i've learned from my hubby that there is no such thing as friends of the opposite sex. all women are seen as potential sex partners. this may not be how all istps are, of course, but mine is a pretty typical, hot, smart, manly man version. mine says, "i don't need any girl 'friends', i have guys for that, what i need is more (you know)" his answer when questionned about his female friends on facebook, is that they have 'friended' him, and that guys have girls on their page, or in general, because of 'scatter theory', which is at it sounds. you befriend lots of chicks in the hopes that one turns into a fuck. i know. he sounds horrible, and i can't really believe i hear him right when he says this stuff, because it's so offensive to my nf sensibility. but he's really just being honest.
i've learned from the one i was intimate with, to
take what he says as the total truth and nothing but. your guys said he's off to school and doesn't have time means just that. as an Ni NF i would read so much into this guy's actions that i never really knew which end was up with him. one moment i was convinced he loved me, the next that i was just a fuck buddy, the next that he despised me, etc., etc. but when i look back at everything, the truest thing i see is that
what he said all along in his little one-liners was really what has panned out by his actions to be the truth.
your guy may indeed be feeling guilty (let's hope he's got that much of a conscience). i hope you can move on fairly quickly from this. NFs just get so chewed up when we tangle with sps. it really sucks. but that's prolly why we find them so fucking hot to begin with. no?
i do think istps make awesome partners (we have a wonderful marriage) but it's only when they've matured a bit or are ready to become a bit less self-centered, or grow a conscience.
can we start a 12 step you-can-recover-from-your-istp group?

pink can be the head counselor. i think it's sorely needed.
haha! was that pun intended?