Hair can grow back, after being lost. The skin biopsy will tell you what is going on in the hair follicles. If the follicles are in a "rest" phase, then there may be therapy to get it growing again. If the biopsy shows no hair follicles at all, he may never grow hair again.
Apparently melatonin is very safe, and 6 or even 12 milligrams is safe no matter the size of dog.
Hormone imbalances (adrenal, thyroid, growth
hormone, sex hormones) all can influence hair growth. One blood test to consider is called an ACTH stimulation test, which we normally use to measure the amounts of the hormone cortisol, before and after giving ACTH. There is a panel, done at the vet school in Tennessee, where they measure about 6 different hormones before and after the ACTH. It is another test I would consider, but to me, the skin biopsy is the next most logical step.
I know it must be so sad to see him without hair, especially a breed that is known for its big coat. I would talk to your vet about a biopsy, and having the biopsy "read" by a dermatohistopathologist.
Good luck, Rebecca