I am 66, a Type II diabetic and I had/have the same problem. Once I found I had low testosterone and started replacement therapy, my muscle loss and weakness halted and I started improving. (I know at my age, muscle loss is not uncommon.)
I hope this helps.
How well controlled are his sugars?
High blood sugar can cause neuropathy and muscle damage. The solution here is excellent blood sugar control with oral meds and or insulin, if required.
Also they should test for thyroid problems and testosterone levels, in case there is other treatable cause.
Is he on any medication to control his cholesterol?