Martin:
Once you are in Windows, you may rename the file that keeps the password:
1. Use Windows Explorer to navigate to the Windows folder (most likely C:\WINDOWS).
2. Look for the file that has your son's user name and the extension .PWL (for example, if his user name is Junior, the file will be named Junior.PWL).
3. Right-click on that file and select Rename from the menu, then rename the file (for example, rename it Junior.OLD).
4. Now your son will be able to choose a new password for his user account, if he desires to do so.