The only elements of crime statutory rape are: she was under 18, and that they had sex.
This is not intended as legal advice. It is only provided for educational purposes and cannot be relied upon as legal advice. Further no attorney client relationship is or has been formed by answering this question.
It is sexual assault to have sexual intercourse with a person who has not attained the age of sixteen. That is a Class C felony, with a maximum penalty of forty years in prison. "Lack o consent" is not an element of this offense. Because your son was seventeen, he is subject to being charged in adult court. Having written all of this, just because a crime is committed does not mean that the district attorney will charge it. The district attorney possesses broad charging discretion. As long as the couple is together, and the father is supporting the mother and the child, it does not seem likely that the State would charge him with this felony (though not totally unheard of).
This answer is for informational purposes only. By answering this question, no attorney/client relationship is created. Although the legal information is accurate, it may not be appropriate for your situation. The best way to handle any legal problem is to seek the advice of an attorney.