You have to ask yourself, what would sincere effort on his part look like?
Intensive individual therapy.
Regular participation in a men's only SLAA meeting (sex and love addiction 12 step program. Men's only because he doesn't need to be around female sex addicts.).
Full transparency - giving you passwords to his email, phone etc.
I agree with others, his interest once you fully let go is pretty typical. It may not last the week. But if it does, and if you consider taking him back, don't do it until he has met the minimum requirements above and shown consistent change for at least six months.
One more warning - from someone who successfully reconciled with my husband, only to have him go off the deep end again years later: if there is an underlying character defect, true long term change is unlikely.
