Then you or your son has encrypted the backup in iTunes. This password is 100% whatever you or him put in when you turned on to encypt backups (its a check box in iTunes) and it has nothing to do with that code that was on the iPhone (unless by chance he decided to make it the same thing).
Anyhow, you can guess at an encryption password as many times as you like. Since it's a local encryption, there is no way to reset it. You either get it right eventually and then the restore will complete and you can turn if off in iTunes after that (using the password that worked), or the backup will never be usable without the password.
Keep trying to guess it until you give up, then just move on with setting the iPhone us as new.
Read this article. It has some stuff that may help you. http://support.apple.com/kb/TS5162