Practically speaking, if the other mother's opposition turns into pressure on her daughter to claim statutory rape, your son would be in trouble.
This is a partial exception because, while these parties are protected from felony prosecution, they may nonetheless be charged for sexual misconduct (a misdemeanor), which can incur a sentence of up to one year in jail. There is also a partial exception for consensual sex between a minor who is 11, 12, 13, or 14 years old, and a defendant who is younger than 17. In New York, there is a partial Romeo and Juliet exemption for consensual sex between a minor who is 15 or 16 and someone who is younger than 21. Named after Shakespeare’s young lovers, “Romeo and Juliet” exceptions are intended to prevent serious criminal charges against teenagers who engage in consensual sex with others close to their own age.