18 and 15? In some states that's actually legal.

He should apply for a pardon or expungement or at least apply to get off the sex-offender registry when his parole is up. When it is refused, as it probably will be, he should find others in the same situation and lobby the state legislature to change the law.

As someone else said, a laddered approach is best.

If the law says "16 is legal, but 15 will get you 20" then something is just plain wrong. Ditto if a 2.9-year difference is legal but a 3.0-year difference gets you sex-offender status for any length of time.