DTMFA. You have no idea what will happen with this love interest from his younger days.
Under no circumstance should you have to put up with this. This is incredibly disrespectful to you. He has to break off all contact with this person, give you complete and unfettered access to all his devices and agree to counseling.
Dose this sound crazy? Well, if you think that is something, re-read your OP, OP.