Try a waterpik (at this point I should be getting profit sharing from the company)

I was in the same boat. I had mega phlegm. I would gag on it then vomit because I had a delicate gag reflex.

Tell him to sleep on his side mouth slightly open if he can - I used to put a face cloth on the pillow that way the phlegm didn't pool. I had a glass nearby and I would spit into it when I had accumulated a fair amount.. (gross I know but I hate vomiting so this helped - I was determined not to puke up nutrition that I worked so painfully hard to get down) I had a thing of paper towels nearby too - to spit into and discard.

The waterpik - they have a deluxe version with a tongue cleaner that looks like a spoon. Use lukewarm water - or even the salt water mixture, and fill the tank, on the lowest setting, with the tongue cleaner in place he can shoot the water around his mouth and clean out the guk. It worked like a charm and kept my mouth clean for a bit. I would also occasionally add a little alcohol free mouthwash to freshen. Hopefully this helps.

Hugs.