Use your FX to ass blast him once it's fixed.
Why would he blast his cousin for breaking his paintball gun? How would that do any good? Maybe it's just me, but why would he blast his family for breaking an object?
I honestly think that it is mostly the OP's fault. He shouldn't have lended his "baby" to a person who does not know how to properly use a gun. He should have taught him the basics before lending it to him and making sure he used it properly. I guarantee his cousin did not break the gun on purpose. It was most likely a mistake. Kindly explain the situation to his parents and hopefully they'll pay for it.