I hate to say it, but It seems like Feline Leukemia. Please find a vet to run that test. You really don't want your baby to suffer. I had a cat with that when I was a child and it tryed to isolate itself as it died. I hope your kitty just has a cold, but I seem to overreact to everything. I really am not trying to alarm you. If he loses any weight that is really not a good sign.

Runny nose could be lots of things(just a cold or could be a virus of some kind). Nasal discharge could mean some kind of infection. Hopefully there's no eye discharge... Keep cat indoors in warm, but well ventillated room and get it to a vet. Ongoing nasal problems in a cat can mess up their nasal passages permanently.