Who was the attorney? Get a hold of that person and ask this question to him/her because this should have been talked about and figured out when you were dealing with this issue in court. Sounds like you should be able to get it done.
Schedule a consult with a criminal defense attorney to discuss his options.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
This could be a case of ineffective assistance of counsel. But before jumping to conclusions, meet with the attorney who handled the case first. If that does not help, consult some other defense attorneys in your area.
It depends on some procedural issues as well as whether he got bad advice. How long has it been since he entered the plea? And be careful to distinguish between rape of a child in the third degree and rape in the third degree -- the latter charge does not require an age difference, merely expressed lack of consent. He should first approach the lawyer who represented him, and if that doesn't provide an explanation, he should contact a lawyer experienced in post-conviction relief.