I wonder if he's actually relapsing or just an active addict that uses rehab and temporarily stops to keep the addiction alive. Run the game as long as you can, get caught, go to rehab, get forgiven, start it all over again. Could this just be his cycle of addiction.
Whether he's really trying or not only he knows - whether you should keep trying or not is a decision only you can make. But whatever you decide guilt should not be a part of it - guilt is a useless emotion right now - right now you have to focus on what is the best decision overall.
As far as telling him your feelings while in rehab - I think its the best place to tell them upsetting things because they dont use while they are there and they have lots of counseling and support to help them through. They need to think about what their addiction is doing to those they love. It would be harder for him to get out and then find out how you feel while he has no support system. One thing i'm dealing with right now is the realization of how my AS's problems and feelings have taken precedence over everyone else's problems and feelings. We're all to shove down our feelings and our needs because no one wants to upset him. I'm seeing that now as part of the codie personality - putting my needs and feelings after those of the addict out of fear that he cant take the stress. Its been one more way i've been trying to stop him from using - but its feeding into his selfishness.
There is another option to the black and white of breakup or stay together? Sometimes having a break while someone gets their life together is a good option. It gets the addiction away from us and away from the kids while still allowing you to be supportive in healthy ways. Maybe a transition period before he comes home would be a good option. If he's serious about it then he'll use that time to get his life together and if not then you'll know and you wont have to go through the pain of getting him to leave.