Hit him where it hurts: His hair.
How to do it: Ask him "Did you run out of pomade bruh?"
Most guys spend a shit ton of time making sure their hair is perfect. Whether they go for a clean side part or a more experimental style, ensuring someone that his hair is fucked up will mess up his day and make him all sorts of self-conscious. Think about it: He'll have to rush to the nearest mirror or pull out his phone to check the status of his 'do. Then he'll ask himself "Oh shit, how long has it been messed up?" "How can I ensure this never happens again" Followed by "Damn... maybe I'm just hideous."