How long does a Feline flare up last?

I adopted a 3 month old kitten from the local humane society last Saturday. He was sneezing a lot and the person working at the shelter said that it could be stress and it will go away shortly. By Monday, he was getting worse so i took him to a vet. The Vet said that there is a 97% chance that he has feline and prescribed some antibiotics and L-lysine in a paste form. Since then, the kitten's nose is completely blocked up which forces him to breathe using his mouth. Then this morning, it sounded like he was wheezing and coughing. I was wondering if this all sounds normal and if so, how long it would last. He is still eating and drinking, not a normal amount by any means, and is using the litter box. I haven't seen any diarrhea or vomiting yet. I've also been reading that taking him into the bathroom and steaming it up would help but he really hates it. I just feel so bad for him. I know its a mostly non-lethal condition, but is there anything else i can do for him to help make him feel better? Any info would be appreciated. Thanks.