Help with congested cat?

Hello all! Looking for any advice or thoughts from anyone else in a similar position. One of our cats has been super congested (whenever he is active you can especially hear it) for a few weeks now. I know most cats always have a low level viral respiratory infection (especially shelter cats), but last Tuesday (9/3) I took him to the vet and they said he may have a secondary bacterial infection so they gave him a shot of antibiotics and said he should be better in a week or two. Today, he still sounds just as bad and stuffed up. I also tried having him in the bathroom while I ran the shower hot and it seemed to loosen up a little, but it still sounds almost as bad.

Anyone have a similar experience? I am planning on taking him back to the vet early next week if it doesn't improve over the weekend but I'd rather avoid the bill if this is more of a 'wait it out' situation.