I know that they have rail covers for cribs, but I don't think (at least I haven't seen) any for the sleigh types. I would try, as one reader said, the biter bite, or like another reader said, some material around the rail.
My son chews on EVERYTHING...they are like puppies that way sometimes! The only reason we even give him a pacifier is because he likes to chew on it and we keep one in his crib for that reason. Maybe that would help? Or keeping some teething rings, etc? Good luck to you!