I agree with the posters who suggested that you maintain a good rapport with your son and not view the situation as adversarial. However, I don't think that means you can't help your son to find his way out of the cult sooner than later.
I've mentioned it on this site before. But in case you missed it, a very good book on the subject of helping loved ones out of a cult is Steven Hassan's "Releasing the Bonds: Helping People to Think for Themselves." He, too, recommends building rapport and avoiding an adversarial approach.
I thought the book was very insightful and filled with good suggestions that can be used with members of any cult, including JWs which are specifically mentioned. Whether your son eventually leaves the WT or not, the suggestions in the book will not harm your relationship with him.
If you can't find it elsewhere, you can order it from the Free Minds website.