Sick kitten, very worried

I've been searching the Internet and through this forum looking for help, but I'm very worried about a little guy I found in my backyard. He was about four weeks old when I found him meowing in the hot afternoon sun last week with what appeared to be an upper respiratory infection. I took him to the vet and she gave him two antibiotic shots, an oral antibiotic, and a gel to put in his eyes. The next couple of days it seemed like he was getting better and was eating well. Yesterday things took a turn for the worse. His nose has become so stuffy he refuses his bottle and breathes through his mouth. We are still giving him the antibiotics, but assume he has a viral infection. We've been giving him kitten formula with a syringe, but it's extremely hard to get him to swallow. Even with the formula in his mouth he tries breathing instead of swallowing. We're keeping him upright so he doesn't choke and going slow, but it seems like almost nothing is going down. He's been very sleepy and his eyes are better than they were, but his nictating membrane doesn't pull back, ever. We've been keeping him in the bathroom with a vaporizer going, hoping it would loosen his nose, but there's been no change so far. Right now I'm most concerned with his eating and breathing, but his eyes worry me, too. He sneezes sometimes but doesn't cough. Is there anything else I can do to clear his nose? I'm scared putting drops in there because what if he inhales them (occasionally his left nostril clears and he breathes through that and his mouth)?

Have you called the vet back to see if they have any suggestions. That would be my first step if he were my kitty.

Awwww..... Thank you for rescuing this little fella!

The vaporizor was going to be my suggestion. Given you've already tried that, I agree, the best next step is calling the vet to let them know about how he's doing and find out what they suggest.

My cat Shiloh was rescued from a kitty mill/hoarder. He had the same problem. He mouth breathed for 3 weeks! The vet orginally put him on an antibiotic, then combined it with something else. (sorry can't remember exactly) We were about to give up hope. He was barely eating as anyone who has had a bad cold or sinus infection knows, it's hard to eat with no air circulating in your nasal passages.
Finally, he put him on Zithromax and Halleluah it worked!! If you look at my cat profile, you will see he's a healthy one year old now!