Complex sentence: He recorded his own songs, including his first single "Ask O'l Hank," which featured his future manager John Michael Whitby on keyboards, and who was then a member of Asleep at the Wheel.
Split into simpler sentences: He recorded his own songs, including his first single "Ask O'l Hank," which featured his future manager John Michael Whitby on keyboards. Whitby was then a member of Asleep at the Wheel.