there are countless studies that demonstrate that people can gain strength at any age, and since your client seems to be rather de-conditioned, he should see functional improvement quickly.
As you know, initial gains in strength are due to neuromuscular adaptation and not the result of muscle hypertrophy. Depending on his current conditioning, you may need to take a very conservative approach in his training. According to the OPT model from NASM (which I ascribe to), the first stages of his training be stabilization and strength endurance which usually do not elicit gains in muscle mass. Considering that his concern is decreasing strength, then improving his functioning would probably be the primary goal.
Evidently, you feel comfortable making specific recommendations to your client as it pertains to his diet. I would leave this to his physician or a dietitian to determine what he should or should not eat.
Here’s some guidance that I found…