Welcome to the MedHelp forum!
Generally such wounds that do not heal are called non-healing ulcers in medical term. They could be non healing due to underlying infection, low supply of oxygen to the wound that is reducing the healing process, underlying diabetes, or due to underlying dermatitis. Wounds heal slower in smokers or alcoholics who have been drinking for a long time. So these have to be ruled out.
Also diabetes leads peripheral neuropathy, atherosclerosis etc which make wound healing troublesome. Your son will need a culture sensitivity of wound scraping to know what antibiotics will suit the infection. If there is underlying dermatitis, then this needs to be taken care of. If there is severe atherosclerosis preventing blood supply to wound, then this needs to be diagnosed by angiograms. It is better to get this treated at a wound clinic. Wound debridement (cleaning of wound, removing slough, etc) would be required proper dressing at a wound clinic.Take care!