my sons ms account has been hacked and disabled

Hi Jarkko, I know this is stressful!

If an account's hacked and disabled, the attacker likely triggered security measures.

First, try the Recover your account option on the Microsoft account recovery page, using his email. If he can still get into the recovery email or phone, use those to prove it's his account. If not, complete the account recovery form with as much correct info as possible, such as past passwords or Xbox info.

Does your son still have access to his recovery email or phone?