Complex sentence: He had grown up using the name Hallenbeck, after his stepfather, but when he decided to apply for flight training, he obtained his birth certificate and learned his father was actually named Charles Boyington, and his parents had divorced when he was an infant.
Split into simpler sentences: He had grown up using the name Hallenbeck, after his stepfather. But when he decided to apply for flight training, he obtained his birth certificate and learned his father was actually named Charles Boyington, and his parents had divorced when he was an infant.