ha.. well.. it could've been any drug really.. that brought out his mental illness.. it's just in this day and age.. with cannabis use so widespread now.. the chances are better that the drug the kid was using was cannabis. It's been proven that ANY psychoactive drug can have negative effects.. especially when abused in people that have underlying or known mental illnesses.
He didn't get mentally ill from his abuse of pot.. but i'm sure it didn't help.