Antibiotics are completely useless against most URIs, which are caused by viruses, not bacteria. They may be useful in treating secondary infections, but do nothing to treat the source of the problem.
A better idea would be to pick up some L-lysine from a health food store and mix about 250mg twice a day into some wet food. Feeding a good quality, meat-based canned or raw diet will also help keep the kitten's immune system in fighting form. Ditch all kibble if you haven't already.
If nose congestion is an issue, hang out with the kitten in the bathroom for 10-15 minutes while the shower is running on hot. The steam will help loosen plugged up nostrils.
Some more info to check out: http://www.holisticat.com/uri.html