But I have a compression fracture of one disc and so much wrong with my joints that if I was a horse they'd shoot me (no actual horses were shot to make this post).

See a doctor.
See a PT.

Hopefully find one of each that bike.

Do what they say

Also just a personal tip, find a LBS that has experience with such issues and get a bike fit. Even if his bike is fit for him within fractions of an inch his body is now different.

So once the Dr. and PT check off for him to ride, ride to the LBS