That strikes me as unethical and even depraved. Is that why you have children - so you can run experiments on them that would never be permitted otherwise? It reminds me of B.F. Skinner keeping his daughter in a box, except that story was untrue.
1. I do not think languages that are under-used have less value than languages that are used more. If this man values Klingon, he ought to be able to teach his kid the language.
2. Using your own assumptions, you can make similar arguments for teaching "lesser" languages like Spanish or Chinese in an English speaking environment. I think that is a dangerous and scary proposition.
3. Most children are able to pick up the english language outside of the household (ie. every single 2nd generation immigrant from a non-english speaking country).
Language is a critical part of human development right from the beginning. Depriving a child of this, and by extension of a normal parent-child connection, in the name of some arbitrary curiosity strikes me as perverse.
This can be compared to someone teaching their child their own religion. No one should be able to restrict that because religion (and most human belief, pending that it doesn't hurt others or themselves) ought to be outside the bounds of morality.
The "Klingon community" has no system of helping its members to make friends or find mates, no useful information about the real world, no "payload" of beneficial values to inculcate in its members (as PG put it in "Lies we tell kids"), no inspirational works of art produced by people it recognises as intellectual forebears, etc. That makes it objectively inferior to pretty much any "involuntary" (religious/ethnic/geographical) community no matter how small, and even most "voluntary" communities (e.g. hackers).
What I really find amusing is that this incident is from 10 years ago, but the original link breathlessly reports it like breaking news.
Wired, August 1999: http://www.wired.com/wired/archive/7.08/mustread.html?pg=8