Why has he had difficulty breaking through in his career as a musician?

This is a consequence of the targeting. Music, as you know, is divine, as you shared with him but not the full implications of what this means—it brands all in the arts as being "too human" in the eyes of the interlopers without realizing they are seeing divinity on display. They only know that it helps other humans and adds to their happiness, and that is an affront to their wanting an evil outcome and suffering. Anything that counters that will be seen as "enemy opposition to be crushed," so they have worked to sideline your client and this is quite a severe headwind to contend with.