Has anyone out there ever been told their dog is allergic to flea's?

We have a 3 year old dog who has never had any type of allergic reaction before. We boarded him a month or so back while my hubby was in hospital. A couple of days after we him up from the vet he started gnawing patches of hair off his backside. I took him to vet and was told he was allergic to fleas. Help.