I have the same issue with my bully. Took him to vet did scrapping for mange but not that. Vet said seasonal alopecia. Sure enough for the last 3 years every spring his flank hair will fall out and than grow back in May. Hair is darker where it falls out.
you might contact Mama J with Obie’s Bulldog Rescue Via Facebook. Her senior bulldog Wannie has had these same senior issues and she has her on medication that has helped.