My rat sneezes. Should I treat him?

If its just sneezing after digging or sticking their nose into something, don't worry about it. If there is red crusty stuff around the nostrils and the sneezing is not related to dust or such, then it is best to be safe and put the rat on an antibiotic. Don't bother with Sulmet..Its worthless on rats and will cause internal bleeding in mice.
I've found the best antibiotic for rats to be Tylan. It comes in powder form (commonly sold for poultry) and can be added to water. Put 1/8 a teaspoon in 8 oz of water and change every 3 days. Keep the rat on the Tylan for no less then 10 days and even more then 30 in chronic cases. If the rat isn't drinking enough due to the taste, add a small amount of Equal (tm) sweetener to the water.
Most antibiotics are available through feed stores, larger pet stores and vets, though one can order it directly through mail order. Antibiotics like Baytril (enrofloxacin) however are only available through a vet or with a prescription. Baytril Note: CAUTION: Don't use in growing animals (stunts growth)

KV Vet Supply  
(800) 423-8211
(call for Catalog)
Omaha Vaccine
(800) 367-4444
(call for Catalog)

In severe cases rats often will stop drinking and therefore medicine in the water will be useless. Severe cases should be taken to the vet, unless you have access to prescription drugs such as Cefa-Drops, Gentocin, Amikacin, etc.
Also Bunni-Mycin nose drops are good. (MJC)
Keep cages cleaned every day during illness, to keep the spread of viruses or bacteria down. Disinfect with Parvasol, Air-Khem A-464-N or other anti-viral disinfectant. If nothing else is available, wash cages in bleach and water, then rinse well and sun dry. Even healthy animals should be cleaned and cages disinfected on a regular basis. Note: If you can smell it, then its worse for them and they have to live in it.