Looking at this situation, here are some quick thoughts:
a) Why be offended by the word "myth"? Yes, sometimes in common language, myth gives the impression of being a lie or a misconception. But technically, myth is a broader term and can encompass social and cultural values within it. In this context, calling something a myth does not really diminish the value (we are not talking about scientific accuracy here). I like the definition of myth provided by Elizabeth Vandiver as "traditional stories a society tells itself that encode or represent the world-view, beliefs, principles, and often fears of that society".
Would this father be less offended if the word myth is replaced by "story"? I don't know.
b) I don't know what was the motivation to include the mention of Creationism in this science textbook. It is quite possible the authors wanted to diffuse the tension by mentioning the origin story in the Judeo-Christian (and also Islamic) tradition - and their use of the word "myth" backfired. May be it is better to keep any mention of creationism out of science classroom.
c) It seems that we are talking literally about the creation story here. If we were talking about "scientific creationism", the idea that the world was created in the last 10,000 years and that this claim comes from geological evidence, then perhaps "pseudoscience" would be a better suited term than either myth or a story (in reality, "nonsense" would be far more appropriate)
I'm sure that Fox News will keep us up to date about this non-story.