I'm a new owner and can give some tips from what I've learned. Hopefully some of the more seasoned members will jump on.
From what I know, these can be signs but there should be others as well. Loss of appetite and less energy can show signs of stress associated with mites. It can lead to seizures if it becomes severe and it can be spread to the other little ones if you have any other ones. If he starts showing other signs of stress and continues the scratching, I would honestly pop over to a vet. I have read that shampoos can be helpful but not always and many have had a vet prescribe Ivermectin in one form or another.
I hope it helps a bit. I would have a vet whom knows about small pets on hand just in case if needed. In the meantime, maybe keep watch. Best to you guys. Please keep everyone informed.