Let me ask you why you feel the need to tell him you can get him what he wants for Christmas. Simply tell him to write Santa his wish list and understand that Santa will do the best to make his toys available for Christmas, but he does have to be a good boy also. Have him understand also that there are other boys and girls that may not get anything for Christmas and Santa might have to share the toys so every little boy and girl gets a toy. No need to go over board here.
Let the child make a list,and pick a few items to buy.The older they get,the more it cost and the more they want.Just find a balance,you don't want the child to start feeling a sense of entitlement,or becoming ungrateful in the future.Things are getting better but,we're still in a recession. :-D