Welcome to Android Central! It's very unlikely that your son could have changed your backup password. He would've had to know what your password was to begin with -- do you typically share your passwords with him?
Since the only way he could have changed it was because he knew your backup password to begin with, that would suggest he changed it maliciously, or as a prank. Use your parental authority to get him to change it back. Otherwise, your only other option is to do a
hardware key factory reset from the Recovery Menu (which, of course, will erase all of your locally saved data). After the reset, you will be required to enter your Google username and password, since it will be considered an unauthorized factory reset -- hopefully you remember that!