Honestly, you and your other cousin sound like total dicks. Bullys. Especially, since you don't make it sound like you even like the guy particularily, so you're just picking on an easy victim.
It might be a diffrent thing if you all had been equal friends for years, and even then it would have been a dirty prank.
As it stands he's totally justified in not forgiving you. If you're honestly interested in keeping a good relationship with him, or anyone for that matter, you should reconsider why you feel the need to hurt others in order to elevate yourself.
And if this is a joke thread, congrats, you got a reaction.