Thank you for the additional information. The symptoms that you are describing are consistent with an upper respiratory infection
. These infections are usually caused by viruses and in most cats will run their course in a week or two. They are similar to colds in people and can make cats feel quite "icky"--not wanting to do their normal activities for a few days.
If he is eating and drinking well, then you are safe to monitor him for a few days. If he becomes severely lethargic, stops eating/drinking, or seems to be running a fever, you should consider having him seen as antibiotics may be needed to help him overcome the infection.
In the meantime, some things that can help are keeping the nose and eyes clean (gently wipe away any discharge with a warm, moist cloth). Keep him well hydrated by adding water to any food
you are already feeding. Steam can also help. If the appetite seems slightly decreased, it may be from congestion and sometimes warming the food can help, just make sure it is not too hot.
I hope this information is helpful, please let me know if I can clarify anything or if you have any additional questions!