You should probably ask yourself how you'd feel if you were living together and he brought a new girlfriend back to stay, presumably after having made it clear that he didn't feel romantically towards you anymore. If the answer is that you'd find it hard to deal with, then you probably shouldn't move in with him. If you feel like you'd be ok with it, and could move on, then go for it. Whilst living together might bring you a lot closer together, and allow you to rekindle what you had, it might also make it completely clear, fairly quickly, that you two weren't meant to be together.