Hello,
Well your fears are certainly worthwhile in an outdoor cat, he is more at risk for many more illnesses. Feline Leukemia and Feline Immunodeficiency virus are both a possibility and both are diseases that cause his immune system to be suppressed and could make a simple upper respiratory infection something more serious. Feline distemper is less likely and has different signs.
On the other hand, it is impossible to say that he doesn't have something that may improve with a round of antibiotics. Since he is such a young cat, it would be very worthwhile to at least have him examined before resorting to something as drastic as euthanasia. If he turns out to be positive for one of the immunosuppresive viruses, at least you will know that there was nothing more you could have done.
Good luck with him and please let me know if you have any further questions.
Veterinarian
Practicing Veterinarian with over 20 years experience caring for Dogs and Cats