Husband's iPhone is not syncing all music from my iTunes account

I've had my iTunes account for approximately 20+ years. I had a 1st gen iPod (yeah, I'm that old). I recently got a new MacBook laptop & I've been having problems syncing to my husband's iPhone since. His phone is a 10sx. We have thousands of songs in our library but when I sync his phone to my iTunes account only some of the music shows up.

Any suggestions? I've spend hours the past 2 days trying to figure this out.