MWM,
I am a "Christian Fundamentalist" by your definition -- e.g. I believe that the bible is God's Word and "literally true". Actually a better term than "literally true" is "inerrant". ([long digression here, you may want to skip ahead to the next paragraph] Many of the passages are not meant to be taken literally, they are allegorical, e.g., the parables of Jesus, the book of Revelation. But these are clearly "labelled", as it were, in the text -- the original Hebrew and Greek typically have stylistic differences between straightforward history, poetry, and allegory. It is fairly common, in my experience, for non-believers to think that "fundies" take everything literally when that is not actually the case. On the other hand, we believe that the creation account, Noachian flood, and Exodus of the Jews from Egypt, with the attendant miracles and signs, and the miracles of Jesus and Acts of the apostles, for instance, really did happen just as set forth, because these are written in a historical "mode".)
Now, I believe that your son's rejection of his family is unbiblical and I feel safe in saying that other "fundies" would say the same. Numerous citations of scripture regarding honoring your parents and carrying the Word to the world, not withdrawing from the world (we strive to follow Christ's example to live in the world, but not of the world) have already been given. If the group to which he belongs is preaching this withdrawal from his family (rather than it being a personal issue with him), then, by definition, they are not "Christian Fundamentalists" (another digression here: "fundamentalist" is not a biblical term, it is usually applied pejoratively by non-believers). I belong to neither Promise Keepers nor Dobson's group (I have some theological differences with them), but they would not, I feel certain, preach the kind of things that you have mentioned regarding your son's attitude towards his family.
I would like to caution against making too-broad assumptions about "fundamentalists" and would also counsel against using that term, especially to your son, as it sometimes signals a certain hostility to and/or misconceptions about beliefs which are very deeply held. For example, the parenthesis in your opening post ("thinks divorce is always wrong and the man is in charge and that you need to spank your kids") is, IMVHO (and please don't take this as criticism but as an attempt to possibly provide some of the insight you desire), overly simplistic. We do not teach that divorce is "always" wrong, but rather we follow Christ's teaching that it is wrong except in cases of adultery. It is teaching against regarding marriage vows as less than sacred, unnecessary. (I am once divorced, myself, by the way). We do not teach that the man is in charge, but that the man and woman are equal partners but have distinct and different roles to play in the household (Ephesians 5:21 sets the tone: "Be subject to one another out of reverence for Christ." This teaching was quite radical in the first century, when women were generally treated as little more than chattel.) We do not teach that you need to spank your kids, but that discipline is necessary -- with an admonition: "Fathers, do not provoke your children to anger..." (Ephesians 6:4).
To reiterate -- if his group is telling him to ostracize his family, they are not Christians, even if they call themselves that. It is very possible that he is involved in a cult if they are telling him to do this. But, the people who you seem to believe teach such things, in fact, do not do so, and if he is in a genuine Christian "fundamentalist" group, then he is in error.
Perhaps if you were able to signal a willingness to accept his religious views, if not agree with them (I'm not saying you haven't done this, only that the terminology you've used and assumptions you've made may, unintentionally, have come across as hostile to his beliefs), it might lead to a thawing trend.