[user104658]

Yeah there are obviously different degrees of brain damage - e.g. plenty of ex-professional-fighters, or people arfter having a tumor removed or aneurism, actually have some degree of brain damage that just manifests as things like slurred speech, memory problems, etc. and in fact there might be no obvious symptoms at all. In this case though it sounds like even on the slimmest of chances of him breathing on his own, he would be in a permanent coma or persistent vegetative state which is effectively not even consciousness.