Also, it might be possible to reach him by allowing him to conduct a "bible study" (in which watchtower publications will be studied, along with cherry-picked scriptures) with you. This will let him trust you, and as you learn more about the cult from his perspective, you may be able to raise questions as if they are genuine questions. There are LOTS of questions that cannot be answered (or, once you're indoctrinated, questions that cannot be asked) about this cult.
I also would suggest that you review jwfacts.com for information about the cult to educate yourself. Don't bombard your son with critical information, though, that will do nothing to shake his indoctrination and will only serve to further damage your relationship. Educate yourself, then work on reaching your son's authentic persona - the person he was before he joined the cult. Once you learn to reach his authentic persona you may be able to slowly introduce information that will help him think critically.
Instead of discussing JWs with him, try discussing the mormons or another group that he would agree is a cult. You can explain how mind control works in their group and whether he admits it or not, he'll see similarities with the JWs.
Above all, you need to give him unconditional love and support. The love of the cult is conditional on his being in the cult, yours must be superior and that will allow him to feel the support he will need in order to leave. The cult drills into you that if you leave you'll be lost with nowhere to go, and he needs to know that isn't true or else he'll feel trapped even if he knows it's all lies.