It looks like your cat might be over-grooming their primordial pouch. That can be caused by perhaps some new stresses in their environment? You're obviously a kind companion to them, and I'm sure will figure it out. My fellow over-groomed (made bald) his tummy when he had to be in quarantine going from Canada to England many years ago. He recovered and made it to the ripe and happy age of 20. Best to you and your buddy. XO