One-time XBox account delinking from bungie account for my son

I gave my son my steam account years ago (i stopped playing video games) but at some point I had linked my Xbox, PlayStation, and epic games accounts to my steam account. Apparently you can’t unlink these things as of March of last year, but I never saw a notification about this change coming, so I didn't fix the fact that all my accounts were linked to bungie instead of his accounts. He just downloaded Marathon on steam but it’s auto-playing it under my Xbox account because bungie has everything linked. He wants to be able to play under his own xbox account, but I don't know how to fix it and it's extremely frustrating for him. He just spend his allowance to buy the delux version and is frustrated that he can't play it under his own xbox account. Is there a way to someone can PLEASE unlink the xbox and playstation accounts tied to my bungie account so He can link his OWN accounts? Otherwise, he just wasted a TON of money...
For reference:
bungie account: RoLiHen#1937