Anytime they lose hair, it can take a while to grow back. If there are no new balding patches, dog is not scratching, and the skin is healing/looks healthy. I would give the treatment already given, a chance.
I’ve fostered a lot of rescue dogs, and have seen plenty of different skin issues. I’m not a vet, and tests could prove me wrong, but my money would still be on your pup having a few extra mites causing his skin issues.
Hopefully the last test results will give you answers. I always hate hearing the word idiopathic, when it comes to medical issues with the dogs. Sometimes if we can clear up the issue, we just have to come to terms with not knowing the cause.