I am not sure about using the humidifier on the oxygen concentrator - probably safe if you use boiled water & keep it cleaned out.
Here is a simpler solution you might want to try - Arm & Hammer makes a buffered (neutral) aerosol saline nose spray. It is not at all irritating, and can be used as often as he needs. I find it works in our house, which must remain drier than my body prefers to keep our windows from icing up in the current cold weather. I also use a light coating of mentholatum or coconut oil in my nostrils at night.
Has he tried a saline spray?
Sue