@Chai,
The poor kitty with cold symptoms needs to have Lysine, sometimes called L-lysine. It is available without a prescription and inexpensive at the drugstore or Walmart ($4 for a big bottle at Walmart) It is in the vitamin section. It is safe and antiviral and strengthens their immune system. An adult kitty should get 500mg 2 times a day. It comes in tablets of different strengths, 500, 1000, which can be crushed and mixed into their food, or pilled or mixed w/water and given by syringe or eyedropper in side of mouth. My kitties don't mind the somewhat salty taste. If he does, try starting with a lower dose and slowly increase to correct amt. It can be increased to 1,500 per day for severe cases. Should be given every 12 hrs. (eaten throughout day mixed in food is okay). It is given a few days longer than symptoms. It's safe and they don't become resistant to it or anything. You can give a lower dose to your other cats to help protect them, as well. You can Google for more details about giving to cats.
Also, clean off his nose everyday, if possible. Cats won't eat what they can't smell, and can get a terrible liver disease if they don't eat for a few days.
Blessings to you both...