Reasons to Consider Euthanasia?

I'm working on a story that involves a child that suffered serious brain trauma, causing a coma that everyone feels he can't awake from, which is what brings around the topic of euthanasia. What can cause such a problem though? I originally wanted to have the kid go with his father on a fishing trip in Alaska and there's a problem and he suffers severe hypothermia. but can hypothermia do this sort of permanent damage to the brain to cause him to be in a permanent coma?