rebel,
I don't know how old the boy it sounds like he is sweet and impressionable and loves his parents. I think you know that the JWs have a bit of an advantage over children like this. Children want to be rewarded for doing what's right. The father can capitalize on that need, especially since the boy has only a youthful perception of the JWs. (It may be years before more serious issues can be looked at deeply by him, and by then, you know that the control mechanisms and his own group survival instincts will have been played upon for too long.)
I don't mean to imply that you will sit idly by while he is indoctrinated but this indoctrination can lead to very serious consequences and the measures taken to guard your son should be that much stronger.
The JWs can quite blatantly make a child hate a parent -- and couch that hatred in terms of love. (Having your eternal interests at heart.) At an impressionable age, it wouldn't be fair to the boy to allow him to keep heading down a road that might get him stuck forever. It may even be worse if he's stuck for 18 years and something finally hurts him and makes him realize that he should have never been fooled. He could end up wondering then why you didn't do more. It is akin to sending a child to prison for a crime he didn't commit, and the child finally gets let out and realizes how much of his life has been stolen and wasted. Although the risk is fortunately small, there are also certain life-threatening dangers he may face within the WTS walls that he might not face outside. The biggest one is the pressure to avoid medical care involving blood. Also, the options for good education are minimized. It's tempting to think of the JW morality as making him safer inside the Watchtower "box," but the people who will have the greatest influence on him will have a very narrow view of "success" which minimizes his opportunities to live a full and truly successful and enjoyable life.
If the situation allows for it, I would have a lot of private talks with him (your little secret) designed to make him think about how people get fooled by controlling people, politicians, presidents, dictators, religions, etc. Believe it or not, a lot of the material he needs can be selected from right inside the Watchtower and Awake magazines, even the Bible itself -- if you are looking for a source that he considers unimpeachable. That might be a lot slower route than a few choice "apostate" Web sites or books, but it might fit the situation. Will he and your husband respect your right to talk about religion with him? Can it be done without downgrading him or his father for their religious choices?
Personally, I'd start with a route that discusses advertising and propaganda. Calling something "The Best" or "The Truth" can be seen as very similar in terms of marketing and "mind-share". You can tell him about the features of controlling religions like the Moonies, Scientologists, Mormons without ever mentioning the JWs (unless he asks !!). The Amy Smart case has brought up the perfect opening to discuss whether there is a such thing as "brainwashing" or if it is merely a very high level of zeal/fanaticism for an idea or religion. The "religion of Capitalism" works through repetition of phrases on TV, for example, that you aren't supposed to think much about, but that you hear week after week after week.
If he is school-age, there are gong to be many opportunities to point out the parts of stories or articles that highlight the importance of breaking out of a rut, thinking independently, freedom from all forms of slavery. If one parent shows a healthy respect for ideas and people that a JW parent would not likely say anything good about, that is going to leave an impression. If disasters happen in parts of the world and someone is apt to worry only about JWs there, you can make it clear that you are concerned about the others, too. I think a parent on the lookout for opportunities could even take a vocabulary list and sow seeds of freedom. (Can you say "myopic"? What does it mean to be open-minded? etc. etc.)
You may not be in a good position to take it to the next step yet, but if you are sure he is getting the point about OTHER groups and religions and organizations, then I wouldn't hesitate to drive the point home about the dishonesty of the WTS. The JWs are hypocritically anxious to point out how no one should want to be involved with any religion or organization that has been dishonest, or that has covered things up. I'm sure this could drive a wedge where you don't want it, but your opinion has to be valid, too. If a son grows up where only one parent's ideas are respected, it's a disservice to the child's own ability to grow up and form happy family relationships.
Gamaliel