Here's what I'd do...just from personal experience I know it works like charm. I'd suggest you find a way to get his response on video too...
1. Go to some mexican joint & eat a bunch of stuff that'll make you shit, and shit hard...I'm not talking drop a pellet--ahh type of turd...I'm talking MobyDick confetti diarrhea!
2. Catch him going into some place, house, friend's house, school, whatever... Hop up on his hood & power-shit all over the front window of his vehicle. If you have the squirts try going from one side back to the other.
3. Watch him flip out when he finds his car window shit covered....what's he to do? He's either got to wipe it off by hand, or turn his wipers on & smear it like a mother all over the place!
By far, that's the best prank I've ever pulled....has never failed me yet.