It probably took a very long time before he was confident enough to leave. It'll probably take just as long if not longer to change his mind again. The more positive and attractive you remain the better the chances he changes his mind sooner than never.
If he starts asking for sex or saying how he wishes things were ok just remember this:
"I agree with you that this is hard, in fact I don't want things to be like this, but this is the path you are choosing".
Don't say it sarcasticly or in spite but with melancholic resignation. (make sure the feelings are true first).
