If it's a new colony, the male could be sneezing due to stress at the change of environment. Aspen is also dusty so that could be a cause as well. (at least all the aspen I have used I find quite dusty, more than pine is anyway) If the sneezing doesn't stop in a few days, then I would either get a new male, (course, if the females are already pregnant, he may wind up eating the babies, unless they have been pregnant only a short time, he may not realize they are not his by the time they have the babies. I really don't thik rats can keep track of time that well).
Another posibility is the male is actually allergic to the aspen. Some rodents are more sensitive to certain bedding. You could try switching to pine or just newspaper (though it would smell more) for a few days and see if the male continues sneezing (I would wait for a week to see if he stops sneexing on the aspen, just in case it's just stress from being in a new environment). If neither of this works, then it could be a food allergy, watch for excessive scratching, loss of hair and health deterioation. Finally, it could be a respiratory infection or other commong rat disease (myco..something)