Honestly, your best bet would be to take him to the vets and whether mites are seen or not, to just treat for them anyways, so you can at least rule that out as a cause.
I brought my hedgie recently to the vets to get treated for mites. There were no signs of any mites on him, the vet didn't see anything, but we treated him with revolution anyways.
Also, what kind of litter are you using? And has there been any recent changes in his diet? Hedgies, like humans, have allergies. It could be that it's his bedding, considering it seems that his underbelly is affected the most.
I brought my hedgie recently to the vets to get treated for mites. There were no signs of any mites on him, the vet didn't see anything, but we treated him with revolution anyways.
Also, what kind of litter are you using? And has there been any recent changes in his diet? Hedgies, like humans, have allergies. It could be that it's his bedding, considering it seems that his underbelly is affected the most.