This really doesn't sound like mites. The little ones sounds as if he may have Cushings disease with a skin infection on top of this. There is a section on Cushings in the healthcare forum. It could also be some type of skin cancer which has started to break open and is now infected. You have tried your best and he does sound quite ill. Unless the vets have actually found mites treating for this can be pointless.
They do enjoy babyfood - something like the little jars of Heinz chicken and pasta. Our like ready brek too and maybe a little cooked mashed veg would be enjoyed
Cushings and Treatment