Jilly, the air conditioner in your home can drive the humidity down pretty low, which may be contributing to your son's stuffy nose. There's nothing wrong with running a humidifier in his room to bring it back up to a reasonable level. And, the humidity during the winter is naturally low, so a humidifier will help then too.
But none of this explains why he's developing an occasional fever. Maybe he has allergies? I would think that would possibly cause some temperature elevation. Does his stuffiness seem worse when he's lying down in bed and seem to clear when he is up and about? If so, maybe he is allergic to dust mites and you need a cover for his mattress.
Perhaps considering both these ideas will help keep his sinuses in better shape and help keep him from contracting so many colds. Dunno.
Good luck!
S.