Hi. I am 4 months out from what you and your husband have just been through ( Stage 4a, left tonsil, 3 cisplatin, 7 weeks radiation).
The mucus will last about 2 or 3 weeks. Have him try to drink plenty of water, this will help thin out the mucus, keep him hydrated and help maintain his swallowing reflex. He is going to be miserable for these 2 or 3 weeks, but after that improvement is quick. If he has pain, ask the doctors about medication, my doctor gave me a liquid form of oxycodone which not only helped the pain, but also the coughing from the mucus. I also found that sleeping without a pillow helped keep it from choking me while I was trying to sleep.
See if you can get some Aquafor for his neck, try Walgreens, also for dry mouth, the Biotene and Stopper's products are good. You can also get a prescription for Salagen to help saliva production.
Try the Head and Neck Cancer area, it's a little more active than this one. Go over there and read some of the past threads, you'll find more info.
Good luck,
Mike