You and you son have separate Apple IDs, right? If not, then I am surprised you are not having more issues than you mention. Here is some information about aliases the may help you sort things out. As stated, it is hard to know what is going on in regard to your problem. See:
iCloud: About your @icloud.com, @me.com, and @mac.com email addresses https://support.apple.com/en-us/HT201771
Using your @icloud.com email address https://support.apple.com/en-us/HT201342.
Note that If your Apple ID ends in mac.com, me.com, iCloud.com, then you can change your Apple ID to any email alias you have from the set: @me.com or @iCloud.com. You can’t change to a third-party Apple ID.
This is a whole different alias area. But, see Use email aliases on iCloud.com https://support.apple.com/en-nz/guide/icloud-ipad/mm6b1a490a/icloud