Legal Question in Criminal Law in Texas

Sex between an adult and a minor is statutory rape, even when there is no force or abuse. Rape is sex with someone who hasn't consented. By law, minors are unable to give consent. Any consent they try to give is invalid.

That the girl's parents knew what was going on does not help your son. In fact, it might have justified prosecuting them.

Whether and how your son can fight this depends upon what happened with his case. Your question says only that charges were filed, but it implies that he was later convicted. If so, when did it happen and what was he convicted of? What was his sentence? Has he completed it? Without this information there is no way to offer you any guidance.

Please bear in mind that your son does seem to be guilty of a sex crime. There may be nothing you can do to remove that label. That is how the law is supposed to work.