It could also be a sign of respiratory illness, so you should probably play it safe and take him to the vets first. Once they have certain resp. problems, they can be contained, but not completely cured, and so it's not nice to bring healthy rats home to pick up the infection.
I got my rats when they were older and they came with resp. problems. Unfortunately, now I don't know what to do when one of them dies, because I don't want to bring in a healthy baby and get it infected
I was thinking of trying to talk to a rescue, and get a rat that already had the same illness so mine could have more buddies, but I don't know if a rescue would even take a rat that they would have to quarantine and spend tons of money on at the vets when there are tons of healthy ones needing homes.
I found this was a useful summary for me: