I'm sure he appreciates all that you have done for him Steve.
My current band has stood by me while I've undergone cancer treatment over the last 4 years, I have to admit that it kept me going through the dark times. I looked to quit but they wouldn't hear of it. I have always stated they should use a dep though to avoid missing a good gig or venue but so far I've generally soldiered on.
Maybe one option would be to look at a more part time/smaller scale situation for him to participate in?
As said, whatever you decide I'm sure he'll appreciate your friendship and efforts over the years; good luck going forward.