Might be an allergy or Ive seen some people say it can be exitement. His new environment might be making his nose ich. If it dosent get any better take him to the vet or if he has a snotty nose, is off his food or has a red stain on his nose. There are some handy llinks for rat health on the myspace below.
See I'm having the same problem with my new girl. I've only had her for 2 days, so I'm hoping it's just her taking in all the new smells. I let her out a lot and she's just so curious about everything and smelling everything! Cause she doesn't sneeze in her cage. So I'm hoping it's just new environment.
Plus I got her sister from the same litter and isn't sneezing at all, but she also isn't as curious and running around as much as the first one.
So (cross my fingers) I hope it's just new home sneezes