URI's can turn from bad to worse within a couple hours, what kind of signs are you seeing? You can use reptaid on him, the bottle has different amounts for how much he weighs, its not by age.
I do have experience using Rept-aid, my 6/mo panther had a "popping" noise coming from him when he walked.. which is one of the main symptoms of a coming URI. As instructed on the bottle I did 10days on 2 days off, and found I did not need to repeat it because his symptoms did go away. He began being more active and eating like crazy as well. This was lucky though... URI's come on fast, but within 3 days the popping noise was gone.
I'd use your personal judgement, I have taken another cham to the doc for meds for a URI but that was because I felt it was paramount he get help quickly. It was upwards of 160.00 for the vet visit as well, and I still haven't really recovered from that money lost.
Good luck, I hope your little guy is okay.