Complex sentence: He had originally wanted to be an accountant, like his mother, he was cast as Harold Hill in his high school (Florida Bible Christian School) production of "The Music Man", the changed his mind.
Split into simpler sentences: He had originally wanted to be an accountant, like his mother. However, he was cast as Harold Hill in his high school (Florida Bible Christian School) production of "The Music Man" and with the experience he changed his mind.