Vaseline all over his door handle. It’s terrible. It’s effective. It’s non-harming.
Pull off his bed sheets, and put things like coat hangers and stuff on his bed, then put the sheets back on. When he lays down he will most likely be totally confused.
Make some “Kool-Aid” for him before he comes home. AKA, dissolve a bunch of salt in a glass of water then put red food coloring in it. This one is devastating, and has caused the drinker to spit all over the kitchen multiple times in my household. It also makes them super cautious about Kool-Aid in the future.
If he has a low-hanging fan, remove the lights. Simple. Easy. Confusing at first.
I don’t know, get creative. Everyday things in large quantities being used in places they aren’t normally used can generally be a prank. Have fun now. And if you use any of these don’t point back to me. Please. 😀