I'm going to tell my dd the truth about Santa. Her grandparents are pretty horrified by it. We spend Christmas at her grandparents' so if she wants to pretend that's fine with me, but she will definitely know that it is make believe.
To the OP: Show your son the respect he deserves and tell him the truth. He is old enough and the longer you leave it the greater chance that he really will feel lied to later on. If you ask me, he already does know and he is leading you by the chain in order to keep the gravy train of presents coming but that's just my humble opinion.