plea aggrement broken

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My son was charged with a sex crime in the plea aggrement he would not have to register for megns law while in prison they took him and forced him to sign megns law paperwork for LIFETIME I am furious and cant find help i know the law changed but he was convicted in 2009 five years ago and has had no arrests for Sex crimes since what can i do to fight this? he is 23 Its not fair Please give advice thank-you