Welcome to HCM.
Since he is diabetic his sugars must be in control to keep any other infections in control.But infection itself will increase blood sugars and he already has gangrene
in his leg and lost one.Try keeping his sugars in control. If the amputation
is required then it has to be done or the infection may spread. Dialysis
is also equally important to keep his kidney functioning or you will have to do kidney transplantation
which is again no favorable in this case. So better follow your doctors advice since it is very complicated.