Long standing non-healing ulcers may give way to Marjolin's ulcer which will be dangerous. You need to have a tissue biopsy performed at the earliest. Please attach Photos of the ulcer for reference. It needs to be cured at the earliest. You need medical appointment in person.

It looks like he has a varicose ulcer. A single medication or external application won't work fully. Along with right food element , local leech application, and medication will work well. If possible please post his daily routine with diet going on.

There are so many reasons for unhealed  wound like less blood supply, metabolic disorders. History of eound is also long so it may be little difficult to treat. Ayurveda medicines with diet medicines and treatments like Lepana leech application help in wound healing. But its better to consult ayurveda surgeons for better treatment