It's a chronic fungal and bacterial and parasitic(demodex) infection  @ malassezia dermatitis !! prolong skin issue , u need alot of patience about it...need to go for skin scrapings needed. Apart of that ur saying it's a stray dog near to ur house... So with that in keeping my mind giving u a basic treatment.

From the pics, it seems to be a chronic skin infection. Probably a mixed infection. You need to consult a vet, probably the vet will prescribe oral medicines, local applicants and bath! It will take a lot of time to get corrected, needs your effort and patience

It is a chronic skin disease..you have to get him to the near by veterinarian for complete analysis..It will take time and efforts.With proper medication it can be treated ..

Skin lesions can be due to many etiology. It can be due to pathogens like bacteria or fungus, ectoparasites like ticks, fleas or mites, or it can be even due to allergies or hormonal imbalance. The best way to successful treat a skin issue is to know the root cause and treat them with specific treatment. It can't be cured in few days. It will take its own time based on the issue. Better take him to a veterinarian so that they will diagnosis the condition and give you the exact treatment plan.