You say he has black patches of skin?
It sounds like Malassezia Pachydermitis. My own dog has a chronic case and we have struggled to keep it under control. She has it particularly badly in her ear canals, causing thickening of the skin, leading to recurrent ear infections and ultimately to her losing her hearing.
It is horrible to see them so distressed and itchy all the time isn't it.
You need to get your vets to take a sample and test it for MP. If it is MP regular baths with a shampoo called Malaseb can help, as can treatment with antifungals such as ketoconazole or itaconazole, although we have found vets reluctant to do this for some reason. (It's a well researched and documented treatment you can read a lot about if you Google.)
After 13 years, the only thing we've found that helped relieve the itch and reduce the blackness is high quality Neem oil. It stinks to high heaven, but it works and was the only thing we found that gave her any relief and led to her fur regrowing on her feet, having previously been bald on her back toes for years.
You do have to make sure it is really good quality organic Neem though. We bought a lotion but added extra neat Neem oil to boost it's efficacy.
Other than that, we found going over to a completely natural bones and raw food diet really helped, as she is atopic as well and definitely reacts to commercial pet food. If you can't do that, seeking out one of the top quality pet foods that is gluten/wheat free may really help her. Something like this or a fish and rice variety - but do make sure it's genuinely grain free. We give ours Canagan every now and again as they have to have commercial food when they're in the boarding kennels and it's very good quality with low allergens. Even something like Burns or James Wellbeloved would probably be better than Iams, which although pricey, is full of potential allergens.