Independent from the decision what language to learn, talk to him about artificial life (programming little animats that live in an artificial environment and have artificial genetics). It is such great fun to program little animals running across the screen. IMHO, this is a perfect approach to learn programming:
(1) You can start off simple (without any genetics or physics) and already have fun, then add more and more features to your animats and to your environment.
(2) The concept of object oriented design is very obvious here: each animat is an instance of a class and has methods and attributes.
(3) If you add genetics, your son will develop a natural view on evolution, which in turn will prevent him from becoming a creationist (god willing).