i dunno what to say. my istp said he was okay with me using the GF label, but he still doesn't really seem comfortable with the term due to past-ex issues (and i wonder sometimes if he is over her). he essentially said the same thing as yours (initially), "i don't know what i want right now. i feel uncomfortable with you being my girlfriend since i just got out of a relationship." that wasn't cool with me, but hey, he is istp...thus a slow mover... REAL slow as others on TypeC have pointed out.
we put a time limit on it (fish or cut bait) because eventually i figured out that i liked him so much that i wanted a relationship.
yeah, i think it would be good for you to determine what YOU want it to be. so at least, well, you feel like you have some control of the situation and aren't just leaving it up for him to decide. i could be wrong, but it seems like you are not too happy with the FWB sitch since you like him so.