A.V.
Have him write down his wishes in a letter and then post that letter by the calendar with a post-it saying "do not mail til December 1". Then when December 1 comes, mail it to Santa, but tell him that since he's already written down his list and Santa is watching, he doesn't need to say it every day.
We emphasize to the kids that a wish is a wish, not a demand, so if Santa comes up with another idea, you should appreciate that one,too.