Is there anything over the counter to give my puppy for chest congestion?

monday i noticed my puppy wasn't acting normal turning away treats and food in general, so i took him to the vet. we did a couple different test main one parvo. All of them came back negative for parvo. got him some antibiotics and pills to keep him from puking and diarea. 2 more days past and 15 pounds lighter he doesn't seem any better. so i returned to the vet for another parvo test again negative. wont drink water wont eat but ive mixed a half water half electrolytes and he really seems to like it, yet its only been a couple hours since i tryed this hes kept it down so far.now he seems to have some congestion hes coughing up flem and yellowish snot. i would like to try and make it easier for him to breath please help what can i do??