Hello, thank you for reaching out to Microsoft Q&A.
Before you look to reset your password, double check that you are using the correct email address when logging in.
If this is a personal account, please use the following link to reset the password: https://account.live.com/ResetPassword.aspx
Open this link on an incognito tab and see if you can reset the password.
If you successfully reset his password, and if you’re unable to sign in to the computer or access any Microsoft services with your son’s account, please let me know.