To maybe help loosen up the congestion, you could try running a hot shower for 10 minutes while he's in the steamy bathroom with you. Might have to do that several times a day. If he goes more than a day without eating, it gets serious. Try feeding him really stinky things like sardines or tuna, plain chicken baby food (no onions), or one of the fish-flavours of Fancy Feast, which can be incredibly smelly.
Hope he's feeling better soon!
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler