I heard Mose Allison sing ´7th Son´about a month ago in London, and it´s on his¨"Live at the Pizza Express" cd. In the same concert, he sang another song which contained just one line about a 7th son of a 7th son. Luckily/wickedly, I recorded the concert (for my own personal edification). I´ll listen to the recording and see if I can find this other song.

Callie