Demodex mites are a normal inhabitant on a dogs skin, they become an issue and cause mange when the dog's immune system is compromised and not able to keep them at a manageable level (ie, times of stress or illness). What's most important to prevent the mange from coming back, is keeping the immune system strong - feeding a good diet is one way to do that, and Blue Wilderness is certainly a good food that you can feed him, but you could also pick just about any 5 or 6 star food from this site:
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I don't know how recently you adopted him - but being bounced from home to home is very, very stressful for them, and that would be enough stress to beat down his immune system and cause the mange to flare up.... so hopefully all he really needs is to get settled and comfortable in your home, so his stress is relieved and not taxing his body so much.