My tri boy lost hair in spots his first winter with us. I assumed it was a food allergy, but my vet told me most skin problems are caused by environmental sensitivities. Sure enough, I finally realized his problem started in the fall when the furnace came on; His crate was right beside a furnace grate. When I moved his crate, the hair grew back. All of which is to say it'll take some detective work to figure this out.
It does look like a contact dermatitis, 'tho! From the location of the problem, I'd ask if your B spends a lot of time lying on the carpet or his blanket in the "frog" position. If the carpet is wool, has rug shampoo or deodorizer powder residues, a rash could develop in that area. If he "scoots" across the rug on his tummy, it could even be rug burn…If his bedding has accumulated a lot of dust mites or was washed in strong detergents without sufficient rinsing, or a fabric softener was used that could trigger an eczema-like rash, and licking of the area would make it more chapped.