I am in a similar situation as you although my children are younger. Although its temping to diss the religion, it could make the child very uncomfortable. Really you have to just teach him to keep an open mind, show him as you are an alternative (normal) life style to that one and always be there with good communication, in this way he can see it more from both sides. If an opportunity arises to explain anything, then be ready with facts and logic. Perhaps if the baptism thing rears its head, you can stress the things that JW's tend to avoid discussing, in other words the implications and so on from giving yourself to the WTS.
CS 101