I suspect it's a staph infection. Your vet can take a scraping of this and test for it.
Puff normally has a pink belly but gets dark splotches on it -- they look a bit as if they could be dirt or changes in the skin's pigment -- they've certainly never looked like scabs to me.
Years ago, I took her to our (otherwise most excellent) vet at KSU's CollVetMed Tchng Hospl and that was what was diagnosed and a 2 week course of Cephalexin prescribed. That cleared it up but it returned in a few weeks so we returned to our vet.
Went through that 3X total until I insisted Puff be seen by their dermatologist.
She did a careful interview of Puff's activities and decided the staph infections were probably being caused by our every morning early walks in the nature preserve where dew wetted Puff's belly and pollen & other irritants got stuck to it and attacked the skin.
We went through a 4th round of anti-biotics and then used, whenever needed at the first sign of a return of symptoms, 3X/wk, a medicated Rx shampoo (Benzoyl peroxide) which gets lathered on her belly, stays 15 minutes, and then completely flushed off.
Either flushing off Puff's tummy in the shower and then throughly drying her OR taking her swimming in the lake (with thorough drying) after our walk accomplished very much the same thing.
Don't know if this is what's bothering your Buddy but it sounds very much like it could be.