Obviously, the more those muscles are used, the more chance
of them building up. But he sounds as if he is getting on
in years, and has maybe slowed down generally? How old is he? And what is his general condition? If arthritis is causing weakness in his legs, maybe if you could alleviate the arthritis somewhat, he would be capable of more vigorous exercise?
I would certainly give him a Glucosamine/Chondroitin supplement.