The reason I thought it was fungal is because on that website I linked, the photo of the pigs stomach looks identical to my pigs back. The fur on his back is thin and when I brush that area the fur falls out easily (However, you're right, he never goes completely bald or has red, bleeding open sores). " In guinea pigs, the tufts of hair may be stuck together by a crust of exudate (the skin beneath may exude serum)." This is relatable though.
He does not scratch AT ALL. However, when I scratch his back with my nail gently he bites furiously in front of him (like my shirt) as if it irritates him somehow.
Those scaly bits are NOT spurs. They are flaky, peely skin where as if I peeled one up, it would fall off and underneath would be a sore. I did used to think it was pododermatitis, but he has clean fleece bedding and it hasn't seemed to gotten worse, just flakier. Only recently has those 'spurs' began, which leads me to believe it's fungal rather than pododermatitis.
Chewy has had mites in the past, and I'm certain it is not mites. He does not scratch like he did when he had them. If it is pododermatitis, I'm already keeping his cage extremely clean, he has a 2x6 all to himself, I soak his feet in warm water. I'm not sure what else to do to treat it. I still think it's something fungal though.