Have Avascular Necrosis On The Leg. Waiting For Surgery. Now There Is A Blister On The Leg. Using Infused Cream. Suggestions?

I am a caretaker working with a 65 year old sedentary male, who has avascular necrosis of the right leg. He is obese, but successfully taking off weight hoping for surgery in four months. Five months ago he developed a 2"x4" blister on his leg 2" above the inside ankle. That eventually burst. A scab formed that fell off. It has healed, but he now has recurring 'blisters' on both legs. Although they are smaller, about 1" diameter or less, he is greatly concerned that this could lead to more difficulty. He is treating this with an Rx silver infused cream. He has controlled Type II diabetes although without medications his sugar is about 115. I took him to a doctor this week who has no clue as to what this is. He cultured it for the 2nd time. Haven't yet heard back. 1st time was typical bacteria, but i don't recall the type. We're getting no where and my charge is becoming very anxious. Could you guide me to getting good treatment? Thanks