try using some of those elastic bands(i forgot the name of them) first before going to weights, they have diffrent tension rates on them so if he gets tired of one he can move up, might be safer than weights
I've always known them to be called resistance bands.
I agree with Forrest here, doctor's opinions vary greatly, depending on their background. When I broke my wrist many years ago I was told to stay out of the gym for 6 months to a year, no lifting at all. I of course was back at it as soon as I could deal with the pain, and had no issues with recovery.
On the other end of the spectrum, when I broke my tib/fib last year the doc had me in PT as quickly as possible, and told me to push it within reason, but to sytay off the leg in terms of weight lifting.
Granted, these are specifically injuries and not a disease, but a great example that some docs know their specific craft in terms of surgery and healing a sedinteary person, but they may not understand the drive and needs of a physically active person, nor have they researched the effects of various types of activity/exercise on a persons recovery and over all physical condition. It would be best to find a doctor that works out himself, or perhaps one that does or used to specialize in sports medicine.