My husband has dry mouth from having radiation. What to do?

His tongue sticks to the roof of his mouth while he is asleep. The doctor from the cancer center told him to use swabs. Swabs do not last long. He sips water during the night and he said that gets painful. He has all the benene things. He eats lemon drops and other sour candy and tart apples. He tried pickles. Pilocarpine can not be used for some reasons. Does anyone know what he can do?