Please note that this is Dutch Q&A Forum, any threads should be posted in Dutch. However, I will still provide an answer on English based on your initial response.
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Hello Dimitri De Leye,
I understand how concerning it is to find that your son’s Microsoft account has been taken over and its details have been changed.
Please note that this is a user-to-user support forum. Moderators and community contributors cannot access the account, verify ownership, restore it, or reverse security changes.
If this is a personal Microsoft account, please start with Microsoft’s Sign-in Helper and enter the original email address that your son used to sign in: Microsoft Account Sign-in Helper
You can also follow the official recovery guidance here: Recover a hacked or compromised Microsoft account
If the Sign-in Helper directs you to the account recovery form, complete it using a working contact email address and provide as much accurate information as possible about the account. It is best for your son, as the account owner, to complete the recovery process using a familiar device and location.
If your son is still signed in on any device, do not sign out yet. Change the password immediately, review the account’s security information and recent activity, and remove any unfamiliar email addresses, phone numbers, or sign-in methods.
For privacy and security, please do not post the full email address, password, verification codes, phone number, or recovery-form details publicly. Thank you so much for your understanding.
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