1) Give him a choice, of riding both ways in your car, or you
drive his car to get him home. He's not likely to want to be
driving.
2) Bring a good book for you to read. Likely to be a lot of
boring down time.
3) Eat a healthy breakfast, and take care of your own health.
4) Do lots of listening. If he wants to talk.
5) Tell him, once in awhile, how brave he is, and how much
courage he has.

I doubt the dentists will let you be in the room while he's
having the work done. Maybe? In any case, bring a good book to
read.