I know that you're a "reader" (not many of us around, anymore!), so here's one of the best available coverages (absolutely trustworthy, plain English, comprehensive) of the "cat cold" thing: Harpsie's Website: Cat Flu
Personally, I have real problems with using antibiotics willy-nilly......
You mentioned "a bit
" of L-lysine.......when I get an outbreak here, I start the cat on 500mg twice a day, and isolate the cat in a room with a warm air humidifier. Within a couple of days I have a cat who's back to normal...I'll then reduce the L-lysine dose to 250mg twice a day for 2-3 days and allow him out of confinement on the 3rd day.
Plain powdered L-lysine in 500mg capsules is what I use - from my pharmacy, like this:
Twist the capsule open, sprinkle all or part onto a teaspoonful of "tasty" wet food, mix it in........done! (the L-lysine is tasteless, but you do want to know that the dose was consumed, therefore that dosing protocol)
There are cats who seem to have periodic outbreaks "no matter what", cats with 'once in a blue moon' colds and cats who will start sneezing after a stressful event.........the last two categories are easily dealt with using the method I described. For cats who have outbreaks "no matter what", regular ongoing L-lysine dosing generally keeps the sneezies at bay.
Has your guy always
had this condition.....or, did it come out of nowhere and is stubbornly sticking around?