The best thing to do is consult with an experienced criminal lawyer. It would appear he'll have to get a copy of the transcript of the minutes of his sentencing where it says he has to be on the sex registry for 12 years. If that is in the sentencing, then, unless the state legislature has changed the law to extend sex registering from 12 yeas to life, he should make a motion to the sentencing court to end his registration requirements after he does it for 12 years.
Speak to the lawyer to check on the nuances of the situation.
the only way this can be changed is to reopen the case to determine if he meets the exception for him not to register. He would need to contact an experience criminal defense attorney.