Phlegm and Saliva filling in the mouth after tongue sarcoma

My husband had a sarcoma of the tongue and throat He was diagnosed in 10/20 and has been cured by radiation and immunotherapy. Now his mouth fills with mucus in great Quantities preventing him from being able to swallow food and also affects his speech. He has been on a feeding tube for over a year. The doctors do not know how to stop the flow. Next step to try is hyperbaric. Any one out there that has success with dealing with this problem.