I say change the locks, but change them with the same brand lock. The only way he's gonna know is if he's going into your apartment and now his key doesn't work.
Then the conversation can go like this:
LANDLORD: Why did you change your locks?
TENANT: Why are you trying to get into my apartment when I'm not home?
I'm sure he'll say something stupid like he smelled smoke, but at least you'll know what's up.