So is comet a Robo? If so I would leave the sand bath in - robos love their sand baths. It may be nothing you can do - some hamsters do lose their fur. But how old is he? The mites theory doesn’t sound quite right somehow. If he’s young it could just be a spring moult
. I had that with Newt who went bald and patchy. It all grew back though, helped by linseeds. I think linseeds are the main thing for coats.