Go to the local auto store and buy one quart of brake fluid.
Go to the local hardware store and buy one small ( 1" to 2" ) paint brush.
'Write' your favorite slurs or curse words on the hood, deck lid, and down both sides of his car using the paintbrush to apply brake fluid.
Next time he goes to wash off his car (or to specifically the brake fluid), it will neatly remove the paint (just where you painted) all the way down to the primer.
Its so funny to see someone drive buy with large 2ft high letters on the side of their car spelling " stunninghunk" !!!
Just dont' get caught or YOU will be paying the $5k plus to get their car repainted.