How long they've been seeing each other isn't relevent. Just because the parents have been aware of the activity doesn't negate that it is still statutory rape. What you CAN do is get him a good attorney, remind him to ONLY discuss his case with his attorney (not family, friends, HER, etc.), and listen very carefully when the attorney lays out possible diversion pleas. Sex offender registration requirements are a possibility.
#1 lesson: The only person who can give YOU legal advice is YOUR attorney