Take him to the hospital, do not give over the counter medications unless you know that it won't interact badly with his prescription meds.
I know somtimes my dad since he is still able to walk and sit upright will sleep sitting in a chair when he has chest congestion. Laying down made him cough or choke. But I urge you to take him to the ER, I work in a medical clinic and a build up of mucus can choke him compleetly if it can't be cleared in time.