MS account of my son hacked, how do I at least block the usage. Let alone restore it?

I'm sorry for the trouble you're having. The only way to recover a hacked account is to follow the steps provided by Microsoft Support;
https://support.microsoft.com/account-billing/how-to-recover-a-hacked-or-compromised-microsoft-account-24ca907d-bcdf-a44b-4656-47f0cd89c245

Unfortunately, if the above method doesn't work and the email address was replaced by the hacker, there's no other effective method to recover your Microsoft account, and Microsoft Support won't be able to help either.

However, you can contact Microsoft Support to request that they suspend your hacked account.

If you'd like to contact them by phone;
https://support.microsoft.com/topic/customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2

Follow these steps to chat:
https://support.microsoft.com/home/contact?SourceApp=smc2&ContactUsExperienceEntryPointAssetId=S.HP.SmcContactUsLanding

Type "Microsoft 365 sign-in issue"

Click the "Contact Support" button

Microsoft 365 & Office > Billing and Account Profile Questions > Confirm.

Select "Chat with a support agent in your web browser."