There are saliva substitutes which are prescription so ask his doctor about this. I have luck with Xylimelts every night and the /biotene spray or gel for daytime. I also keep an ultrasonic humidifier right beside the bed to run at night. Maintaining hydration during the day is also very important. Drink lots of water and even electrolyte solutions, as sometimes they hydrate better. This night time dryness is torture but is usually improves and these things really help in the meantime.