Thanks for the screenshots power, I've removed them from your posts just to avoid sharing even partial account details on these public forums.
So to clarify, it looks like you do not have access to the EA account where that Xbox is linked. We cannot make changes to someone's EA account for anyone but the EA account owner. In this case that would be this other individual.
If he no longer has access to the account either, he will need to get in touch with support to verify his account information before the support team can help make any changes, including unlinking any other accounts that have been connected to that EA account.
I understand you're saying the Xbox account is yours, but we cannot verify ownership of other platform accounts as we'd have no access to their systems to verify any necessary details. The only accounts we're able to change would be EA accounts. If this other EA account is not registered to you, the registered owner would need to get in touch with support before any changes can be made. Unfortunately if they do not or cannot, we can't just make changes to someone else's account, sorry.