My prosthodontist told me early on that after radiation, I should use my fluoride trays DAILY for the rest of my life, if I want to keep my teeth. If that was not the advice your husband received from his dental professional, my recommendation would be that he ask his medical team for a referral to a dentist who has experience in dealing with the specialized problems that occur after radiation for a consultation. Just like you would not expect your family's general practitioner doctor to be a specialist in oncology or cancer treatment, it could not hurt to have one visit with a dental specialist.
I've been using my fluoride trays daily now for 5 years.
Dental health is a serious concern after radiation TX due to decreased saliva, dry mouth, and radiation damage to the teeth roots and jaw tissues. For example, Extractions are a big No No without Hyperbaric Oxygen treatment.