Simply soak his chair in water. When soaked there is no evidence it is wet until it is used.
Glitter. Pour it in his desk drawers. It is the gift that keeps on giving. He will find glitter particles forever. On his crap, stuck to his face, etc. Never really gets cleaned up.
Brute cologne. Just a little bit on his arm rest, chair, etc each day. It adds up very quickly.
Go to a blank spot on his desktop
Right click
go to "Properties"
click on "settings"
click on "Advanced"
go to "graphics driver"
click on "graphics properties"
click on "rotation"
go from there ...
Does he use an umbrella? Paper punch dots into an umbrella are great.
If he is a hunt and peck typist, reverse a couple of keys.
Icy Hot. Lip of the coffee mug. Little mean though. Done it. Works on TP also, but that is nearly impossible at work.
If he has a desk phone wire the cord short or glue the receiver down.
Reverse his desk and everything on it.