Hi @worrywife As I read the previous comments, one thing stands out. This takes time. Radiation specifically more or less roasts a person from the inside out. It takes many months for this to heal.
In the meantime, consider having his thyroid levels checked. The reason for this is that often the thyroid is bruised or otherwise damaged by the radiation. Any significant variance in the thyroid TSH and/or Free T3 T4 levels results in lethargic or overactive behaviour of the patient. This simple blood test is often overlooked by healthcare after radiation but certainly has been a factor in many patients response to therapy including my own.
And finally, what your husband has just gone through, basically a near-death experience, can result in a basic shutting down of the outside world while he reflects on his life, your life, and the world around him. While for years he was in control of his life, that suddenly is or was no longer the case. Basically a form of PTSD, he has gone through something he has yet to wrap his head around. Usually the patient will work this out himself. For now, he is scared. "What if the cancer comes back?" "I should be better by now." "There is a new sore in my mouth and it could be cancer!" (it's likely not). "What if my next checkup has bad news?" Believe me, try living with those thoughts rolling around in your head and you will quickly understand his societal drop out as of late.
He is welcome to private message me for support. I have been through this and more. If not, I understand and don't push him. My advice: see if he shows improvement week to week, not day to day. And as my friend Winston used to say, "Courage!"