90 day error Help! New iPad for son all of his music is on my iTunes account.itunes & app have my account but no access to music.

I do not have auto downloads on any device. He is off to uni and has his own Apple ID now but all the music for the last 6 years has been bought on my account as it is for most teenagers with no credit card. How does he get access to his music on the iPad I purchased for him? Why is this so difficult? Apple clearly wants families using as many devices as possible, do they not understand children grow up? His iPad is registered to his Apple ID. The music and app on his iPad have my Apple ID as recommended on the endless support forums I have read to date. We do not have iTunes Match on any device and we do not use home sharing and given everything I have read both of these have huge issues with the same 90 day error. I bought the iPad I bought the music it should be simple to verify this is my account. I am so frustrated as this must be a common issue, kids grow up so this must happen to every family. It's teenagers who are buying much of the music are we supposed to get them their own credit card and Apple ID at age 6? What is the simple solution to get rid of this 90 day insult to Apple consumers?