Forgiveness is good, but this must be tempered with you being honest with yourself and your son...
I just don't know how this guy can flip between waving a "bacehlor pad" in your face a few months ago to the song and dance he's offering you now...
The suggestion of you holding back isn't to PUNISH HIM, its to TEST HIM...
DBing doesn't mean accepting a spouse back just because they decide they want to... You need to consider his motivations and potential for change...
You pointed out he isn't doing much to win back your trust yet anyhow...
I think you should just let the Therapist work with him for a while longer before you throw your hat in... If your H gives up after only a month or so at this then he wouldn't have made it long term anyways..