I'm not a doctor, just a patient. I know of patients that were slow to gain weight, myself included. In my case my thyroid was bruised by the radiation, which throws off the TSH levels and causes all sorts of angst with the endocrine system. Your husband will likely get a blood panel to see where the issues lie. Keep at it. We are our own best care advocates. We really don't want to learn what is wrong with us at the autopsy.
P.S. I was 99.9% sure his scan was nothing to worry about. It's that gosh darn 0.1% that dogs us for years and years.