Hi there! Sorry to hear about what happened with your son's account.
If the hacker changed the email address or security information and you can no longer sign in, the best next step would be to follow Microsoft's guide for a hacked or compromised account, rather than only submitting the ACSR form:
Microsoft account hacked or compromised
Just to clarify, we're fellow Microsoft Community members here, so I also don't have access to your son's account, security information, or Microsoft's internal account recovery systems. Only Microsoft can review the account and make changes to it.
Hopefully the recovery steps above help you get the account back.