Hi, LouRay. I'm 5 months out of treatment for tonsillar cancer, and I had the coughnig and gagging, too.
Try some Mucinex to help thin out the mucus in his throat. Talk to the docs about getting some anti nausea drugs to help the gagging. Drink plenty of fluids, I found orange or mango gatorade tasted better than plain water. Experiment with different sleeping positions, I found that sleeping without a pillow helped me, but I have heard that sleeping upright in a recliner helps too. Some herbal teas like "Throat Coat" may help. Get a humidifier, especially if you live up north where the heat is on a lot.
I had a lot of pain in my throat from radiation and was given a liquid form of oxycodone, which helped not only the pain, but also the gagging and coughing. Maybe he can get a prescription (codeine based) cough syrup?
It never hurts to ask the doctor(s) for something to help, mine were, as most are, willing to do what it takes to alleviate the side effects of the treatments.
Best wishes to you and your husband, I hope some of this helps.
Mike