He still committed sexual assault of a child (you) even if you agreed to have sex with him because he was TOO OLD and you were TOO YOUNG under the law to consent. I do not see how you can really help him. He would have been convicted of the sexual offense regardless of which way it went - forcible rape or sex with someone so young. (It is not right alleging forcible rape - that probably resulted in a higher sentence - but it does not affect his registration requirement.)
If he has a lawyer, I would talk with the lawyer because that is the morally correct thing to do. But. I would not just do what he requests but I do not see how that can help him. He was under a legal obligation to register regardless of the way the sex occurred.
Although I have answered the question to try to help you, you should consult with a lawyer in your area in person on the matter. In addition, my answer does not establish an attorney-client relationship between us.