Was your boyfriend ever on the lease?  Did he return the keys to you?   If he tries to move back with the help of the local law enforcement (rather doubtful as they don't like to get into civil matters) you would need to prove to them that he voluntarily vacated the rental unit (or your home if you own it).

It's not unusual that substance abusers that go on binges (no matter the drug of their choice) will attempt to talk their way back into living situations once they run out of funds (or substances) to keep them high.

Gail