Complex sentence: Frontman Austin Garrick was inspired to write the song by his grandfather, whose reference to Sullenberger as "a real human being and a real hero" became the song's refrain.
Split into simpler sentences: Frontman Austin Garrick was inspired to write the song by a quote from his grandfather, who spoke of Sullenberger and the incident. Garrick's grandfather referred to Sullenberger as "a real human being and a real hero", which became the song's refrain.