You are right. What you are missing is that a 17-year-old is an adult in WI for purposes of committing a crime. So the 17-year-old could be charged with a class C felony, sexual assault of a child. (That is WI statutory rape, so consent of the child is not a defense.) The maximum penalty is 40 years prison. Sex offender registration and sex offender probation rules are also a included in the law.
He needs experienced legal counsel right away.
My answer to your question is based on the facts that you provide in your question. Additional factual details about your situation could change my answer completely. The law in inherently uncertain and always subject to change.
This also could include lifetime registration as a sex offender. He shouldn't talk to the police or make any admissions involving the intercourse!
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