The easiest solution is to reset the password ***** your son' Apple ID, this can be done by going to Apple ID here.
Type in the e-mail address your son would have used, then follow on the screen prompts.
Once the password ***** been reset, you can now use this new password ***** unlock the phone.
Please let me know if you need any further assistance.
On the page there is a link which says 'forgotten apple ID' if you click that, then type in your Son' name and e-mail address it will send the apple ID to your via e-mail.