Hamster Hair Loss Mites

Campbell Dwarf Hamster Hair Loss?
My Campbell dwarf hamster Opie has had some hair loss on his chest area and by the joints of his arms and legs. The hair loss doesn’t go all the way down to his scent gland, but all the hair loss is on his underside. I don’t see any mites, but I do see him bite/scratch it. I don’t know what it could be. He’s going to be around 2 in February, but he doesn’t look old because his fur is still the same color it was when I first got him (not grayish around the face).
I just recently changed his chinchilla sand to corn meal. I hope that’s okay. He’s not eating it, just rolling around in it sometimes. Other than that, he has carefresh bedding, and I haven’t changed anything in his cage other than the chinchilla sand.
I have heard that it could be vitamin deficiency too. What foods are high in vitamins that I could feed him? Could it just be old age?
My hamster had hairloss problems, mostly around one of her back legs. They have drops you can put into the water for nutrients, or I just got vitamin treats, they kind of looked like green cheerios. The hair doesn’t grow back, but it should stop the future hairloss.
