Keep a close eye on him. He may be sneezing because of dust in the feed or,bedding, but a respiratory infection such as infectious bronchitis might be a problem that he has caught from another carrier chicken recently. Make sure that he is warm if he appears to be chilling. IB can spread throughout a flock quickly. Sneezing and clear nasal drainage are the main symptoms, but little chicks may be more at risk. I would contact the seller to tell them about the problem. Here is a little reading about IB:
Infectious Bronchitis
Synonyms: IB, bronchitis, cold
Species affected: Infectious bronchitis is a disease of chickens only. A similar disease occurs in bobwhite quail (quail bronchitis), but it is caused by a different virus.
Clinical signs: The severity of infectious bronchitis infection is influenced by the age and immune status of the flock, by environmental conditions, and by the presence of other diseases. Feed and water consumption declines. Affected chickens will be chirping, with a watery discharge from the eyes and nostrils, and labored breathing with some gasping in young chickens. Breathing noises are more noticeable at night while the birds rest. Egg production drops dramatically. Production will recover in 5 or 6 weeks, but at a lower rate. The infectious bronchitis virus infects many tissues of the body, including the reproductive tract (see Table 1). Eggshells become rough and the egg white becomes watery. (See publication PS-24, Egg Quality, for other causes of poor egg quality.)
Transmission: Infectious bronchitis is a very contagious poultry disease. It is spread by air, feed bags, infected dead birds, infected houses, and rodents. The virus can be egg-transmitted, however, affected embryos usually will not hatch.
Treatment: There is no specific treatment for infectious bronchitis. Antibiotics for 3–5 days may aid in combating secondary bacterial infections. Raise the room temperature 5°F for brooding-age chickens until symptoms subside. Baby chicks can be encouraged to eat by using a warm, moist mash.
Prevention: Establish and enforce a biosecurity program. Vaccinations are available.