hmm..we actually went thru the same situation. everything. like the part were you still hang out together, and he tells you about what's happening to him and this other girl... and we also came at a point when we looked and acted like we were back together, but the thing is, there wasn't really any agreement on the said topic- it's like i was ready and he was ready, but he still didn't want to get back together... so what we both did was talk about it thoroughly, and agree that BOTH of us are going to wait for a while. but at that moment, i asked him upfront whether he does want to get back together or was just with me 'just because' he's waiting for something else. I told him i didn't want to be a rebound girl. and he clarified that he does want that also, he does want to get back together, but not immediately after breaking up with the other one.
so at the time being, i'd suggest to ask him. face to face. no walls. ask him straight on what's he's planning to do, with your relationship. and then work on the answer. clariy points, tell him your thoughts, and ask him about his also.