Why did you break up?
Is the reason for the break up still an issue?
21 day rehab is not a magic bullet to cure alcoholism. The really hard work is staying sober after leaving the confines of the rehab facility.
I think you should trust your gut. Your gut tells you that dating him is not a good idea. Yet he keeps trying to convince you otherwise. You understand he is minimizing your wishes to only be friends after rehab, right? Is that how you want your friends to treat you?