How old is he? How old are you? His parents might not take you seriously, maybe you need to go to another relative and talk to them about this, and they can get through to his parents. Abuse with animals can lead to abuse on humans, make sure you tell them that...I'd be a little worried too.

My little cousin, I think he was 4 or 5, came over my house and found a frog, and he was showing it to me from behind our glass sliding doors. He put it on the glass, squished it and dragged it down the glass. Then started laughing and running around going "froggy dead!". He has been diagnosed with several mental disablities, since then. His parents ended up getting chickens (in a chicken pen) and they work with him on it, but he is ALWAYS supervised around them. He's gotten better. Definately talk to someone about your cousin.