Is "humble myself" idiomatically sound?
Your father probably has a charming way with words, and although "humble myself" as he uses it is idiosyncratic, not idiomatic, he has not stretched English to its elastic limit -- to use the term from your bio. He should continue using this phrase, because it pleases him and no doubt his listeners.
idiosyncratic, from Dictionary.com:
pertaining to the nature of idiosyncrasy, or something peculiar to an individual: The best minds are idiosyncratic and unpredictable as they follow the course of scientific discovery