Proper way to reassign installed macOS to another Apple ID?

Solution 1:

You will need to erase the disk to change ownership, not just reinstall macOS over the current install.

You should erase your startup disk in these circumstances:

• You’re selling or giving away your Mac, so you’re clearing your Mac of anything associated with you, your data, or your Apple ID.

To do that, boot into Recovery Mode by holding ⌘ cmd r during boot. Then follow Apple’s instructions for Erasing your startup disk and Reinstalling macOS:

1. Select Disk Utility from the utilities window of macOS Recovery, then click Continue.
2. Select your startup disk in the sidebar of Disk Utility.
   _{Select the disk name, not the volume name indented beneath it.}
3. Click the Erase button or tab. _{If you don’t see an erase button or tab, select the volume name indented beneath the disk name, then look again.}
4. Complete these fields:
   • Name: Enter a name for your disk, such as “Macintosh HD”.
   • Format: Choose Mac OS Extended (Journaled).
   • Scheme (if available): Choose GUID Partition Map.
5. Click Erase to begin erasing your disk.
6. When done, quit Disk Utility to return to the utilities window.
7. Choose Reinstall macOS from the utilities window.
8. Click Continue, then follow the onscreen instructions. You’ll be asked to select a startup disk and click Install.
   _{To verify eligibility to install macOS, your Mac serial number is sent to Apple, and you might be asked for your Apple ID.}
9. Your Mac restarts to a setup assistant after installation is complete.
   _{Complete the setup assistant with your son’s information.}

Completing Setup Assistant is what will assign the OS license to the owner’s Apple ID.

Solution 2:

The only MacOS that's a paid upgrade is Lion. You'll either need to buy a USB key (Apple sold them for a while - legitimately with a license, not the gray and black market installers you mostly see for sale these days) or call Apple Sales / AppleCare and buy a redemption code for your new account.

Once that's over, you can upgrade past 10.7 for free from the App Store as well as get free redemptions of iWork and iLife suites. Apple doesn't reassign licenses - so you need to get it again paid or redemption style in all cases where there isn't family sharing (which is only for iOS presently).