It sounds like a rash that diabetics get when the circulation starts to slow up in the legs/feet.
Diabetes is actually in the long run a circulatory disease...the constant high blood sugars or low blood sugars and the fluctuation starts to wear away at the blood vessels...starting in the extremities and the eyes (that is why many diabetics suffer from glaucoma). The smallest blood vessels go first and then it moves in ward towards the body mass.
He needs to get it checked out ASAP...because if it is or becomes infected he can end up losing the limb or his life from a blood infection.
I speak from experience...my mother was a type I diabetic from childhood and was always very very careful of her feet...because she knew cuts or other things would not heal as fast because of the lifetime of diabetes. She ended up falling and breaking her leg at 57...and it finally had to be amputated because there was not enough circulation for it to heal itself.
He may be scared of what the doctors are going to tell him....that he needs to lose weight (he knows that already)...that he is diabetic (probably from the rash you are describing)....but the doctors are going to want to help him and hopefully save any lecturing for a later date if a family member goes with him and lets them know how hard it was for you to get him there.
In the end only he can decide if he wants to seek treatment...but the family should let him know how much it will hurt them if he ends up dying early from something that could have been treated. HUGS to you!!