I like simplicity. Pizza grease in your food wasn’t harmful but said “HAHA! I GOT YOU FUCKER!”
I like the grease under the door handles on his car.
Conditioners dont smell like shampoo. He’d know immediately. Unless… you can find one that DOES smell like his/looks like it. Good luck.
If you don’t mind disgusting shit… put a dead fish that is about 2 inches long under his matress, in between the boxspring/matress. Not destructive, and yet… it’ll slowly start to rot and smell. Put it in a plastic bag, so that it doesnt destroy the matress/box spring. You want to get even, not be destructive. Actually, you could probably hide it behind a shelf or something in the bag… that’d probably be safer and just as effective.