adam, you still looking for ideas, put a cinderblock underneath his rear end on his jeep, just enough to get the tires so they arent catching. he'll throw it in drive, and go to accelerate and nothing will happen.
if the door in his room opens into his room, instead of out to the hallway, go to home depot and buy a crapload of bricks. make a wall in his doorway, so he goes to open the door he just sees a brick wall.
rubberband a condom around his tailpipe. get an extra large.
take his shampoo bottles out of the shower, put the shampoo in another container, put jello in the shampoo container, he goes to squeeze it, nothing happens.
buy a blow up doll and fill it with helium. tie it to the back of his car, with a longish rope, hide the rope and blow up doll behind something else.
he goes to drive off it'll float up. and if he doesn't pay much attention he may drive around like that.
i haven't heard of anyone doing this but it seems like it would be funny, to take a bottle of whipped cream, and divide it in 2. take off the air conditioning duct thats facing him in the car. throw it in there. its summer, and it gets hot as hell out, so he probably will get in the car and crank the air conditioning.
also never heard of anyone doing this, but get like 6 horns from a junkyard, wire them up to his reverse lights, so when they come on when he goes to reverse, the horns start blaring. hide the horns inside like his wheel wells.
if he has book covers or keeps his books in a backpack. throw a couple special books such as
Coping with Erectile Dysfunction
Coping with Premature Ejaculation
i think i'm out of ideas. now i want to go prank some friends. haha.