I have to say while I knew them as vocabulary words, I never other then eHell have heard people refer to "introverts" and "extroverts" and it always chafes me on eHell, as I find their usage somewhat obnoxious. The usage of the words implies some sort of pleasure or something in reference to speaking or not speaking a lot.
While the tour guide was perhaps rude in tone, many adults don't expect children to be "extroverts" - which is to say most adults don't care if kids want to, or are happy to, or are comfortable speaking. They expect kids to speak when spoken to and to answer relevant questions whether they are introverted or extroverted. (Which is my issue with the labels - I don't expects guests at a party to be extroverts, I expect them to be good party attendees, one aspect of which is holding down a few casual conversations over the course of the event, just like I don't expect long haul bus seat partners to be introverts, I just expect them to amuse themselves instead of relying on me for conversation for hours on end - for me its about personal social responsibility, not social comforts.)
Whether your son is introverted or not, he'd do well to learn how to verbally communicate effectively in appropriate situations and labeling him as an "introvert" I think is not doing him any favors, but rather enabling bad habits. Introverts still need to answer questions throughout life.
Now perhaps the tour guide didn't actually require an answer but if your son was asked a polite question and he just stood there not speaking for a good length of time while he formulated his answer, that's pretty unacceptable behavior (unless your child is very very young). Even if the answer is "I'm not sure" or "I need to think about it" or "maybe" if its coming across that he doesn't speak at all, that should be concerning. Not answering people and just standing there staring blankly at them is generally considered rude behavior.