I bridle a bit at paying for an extra service
That's completely understandable.
Based on your requirements and the moderate complexity of your device echo system, I don't think there is an obvious or straightforward solution to your challenges. That said, with a bit of effort, it likely is possible to manage your music to your specifications across all of your devices.
If you don't mind sharing this information, I have a few questions for you.
- How many devices are you using to manage your music?
- What is the age and model of each device you're using to manage your music?
- Which operating system is installed on each device?
- In what format is the music created by your son saved (e.g., .aiff, .wav, .alac, .mp3, etc.)?
I would check to make sure you're running the latest version of iOS/macOS on each of your devices. Some older versions of Apple Music may not support the format your son's music is saved in.
iTunes Match gives you access to your entire music library from any device, regardless of whether or not that music exists in a public database. You can rip a CD, add metadata to it, and iTunes Match will ingest the music and treat it as a part of your overall collection.
If you add your son's music to an Apple Music library with iTunes Match enabled, the tracks will be uploaded to iCloud. Any metadata and artwork associated with the tracks will be uploaded to iCloud as well. The same music will then be available to all of your devices for download on a per track or per album basis (from any device).
Music in your Match collection that has not been downloaded will be visible with a cloud icon indicating that you can download it to your current device.
iTunes Match was one of the better decisions I've made concerning managing my music library. It turned an oftentimes tedious and manual library management process into a largely automated exercise requiring little intervention on my part. My entire music library (including non-iTunes tracks), containing some 10,000 songs, is available to any of my devices via iCloud. I download the albums and or tracks I wish to have locally on my device to avoid streaming over cellular or WiFi networks. This arrangement makes the question of which device my music is on almost irrelevant. Any device I sign into has access to all of my music regardless of whether or not I purchased it from Apple.
Apple will also swap your low-res tracks for higher quality files for music you didn't purchase via their service (if the music is available in Apple's library). I've always thought that was a nice perk of the service.
I hope these notes prove somewhat helpful, best of luck to you.