Thank you for the information and your question. This is a tough issue. Assuming that there was no evidence that there was force involved (which is always tough since you cannot control what the allegation is) and he does not live with this family, then technically he would not be guilty of Criminal Sexual Conduct, either felony level or misdemeanor
, since she is over 13 and they are within 5 years age difference. However, there is an obscure law which I have set out below, that could arguably be used if the State just really wanted to go after your son for the sexual encounter. That said, it is not likely used anymore since it essentially says it is a crime to "seduced" and "debauch" and "unmarried" woman. Not even sure why it is still on the books, but there are all sorts of inane statutes that just linger on the books but are never enforced.
One other issue those, is things unrelated to their sexual relationship. For example, if he is with her when she smokes or drinks or does anything illegal, he could be charged with contributing to the delinquency of a minor.
Your instinct is good and one big problem that these young men (usually men) don't grasp is that these are just girls and although they may be physically mature, they are not emotionally mature, and that may lead to all sorts of wild accusations if the relationship doesn't go the way she wants. But, hopefully, that won't happen.
You can read all of the Criminal Sexual Misconduct statutes by going to: http://www.ageofconsent.us/state-laws/michigan-age-of-consent-laws/
Please let me know if you need any clarification. I would be glad to assist you further if I can.
750.532 Seduction; punishment.
Punishment—Any man who shall seduce and debauch any unmarried woman shall be guilty of a felony, punishable by imprisonment in the state prison not more than 5 years or by fine of not more than 2,500 dollars; but no prosecution shall be commenced under this section after 1 year from the time of committing the offense.