I'm not sure if you'd be able to do this (I know I sure wouldn't) but the quickest and most humane method would be to destroy the brain. Is there a vet school or any university you might be able to bring him to? I know when I was in school, we had to pith bull frogs (I know, it's way different, but it's an option that may work), so they might be able to take care of that for no or minimal cost.