It really depends on how old he is, but I think a whoopee cushion might be funny for him if he's like 3,4,5, maybe even 6.
A fart machine in there desk
revenge is good up to a point, if it is just a little prank you want to pull that's alright, but you don't pay evil with evil.
The good old bucket of water on the top of a door prank. :)
pull a nice good prank
you can put jello in his shoes
Whoopie Cushion is always a classic!
no No there was a guy named Isaiah Michael Jackson who thought it would be a good idea to dress as a Gorilla and pull a prank by snactching a kid.
do something funny
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pull a prank on him
U put water all over his head
put glue in someones shoes or put pepper in someones sandwich