"Brain dead," no problem. I drove an ambulance for a living and completely understand that. No brain activity equals dead.
Brain damaged, however, means still alive.
Now, my brother in law was in a very nasty accident when he was a truck driver and ended up in a coma, then a "persistent vegitative state" for several months (he was unresponsive to external stimuli). With intensive theraby he got better, to the point that he is up and about. My sister didn't know him when all this happened, so he obviously is doing pretty good. His condition is similar to someone who has had a medium stroke. Some speech and cognitive impairment, some difficulty walking (but that's more from hip damage than brain damage). He gave me a video several years ago that laid down some good background.
One thing about him, however. He's made it very clear that he doesn't want to go through anything even remotely close to what happened before. He refuses to wear seat belts and he carries a living will (with DNR in big block letters) wherever he goes.
I can understand his POV...that's a hard row to hoe...
You might want to warn him that DNR now includes a feeding tube even if he's on no other "machines". If all he needs is a tube, he won't get it with a DNR.