He likely has an upper respiratory infection that is often contagious to even young, vaccinated dogs. My recommendation is to just make sure he eats/drinks overnight. You can try a nebulizer or humdifier to help him breathe a little easier. If you do not own either device, you can turn on the shower with the door closed and sit near the shower with him to allow the steam to fill the room. This humdified air helps them breathe a little easier. Another treatment is the product "Little Noses" in the children's section of the pharmacy. You can apply 1 drop into each nostril once a day for 3 days only. This will help dislodge some of the congestion if his nose is clogged. Also, make sure he is eating/drinking. You can try to warm his food in the microwave so he can smell it better.
Otherwise, he sounds like he needs antibiotics. I would definitely keep that appointment tomorrow and have him evaluated. And keep an eye on the other dog so that he doesn't get sick too.
Best of luck - please let me know if I was able to help you. - A Wiliams, DVM