Is it legal for a 17 year old boy to have sex with a 15 year old girl?

The age of majority for both states is 21. Your son and his girlfriend are both minors and therefore statutorily unable to give consent. This does not preclude the girlfriend or your son from filing criminal charges for such things as assault, battery, etc after the fact.

The age of consent in NY appears to be 17 and for PA appears to be 16. That means that sex with a person under 17 in NY or under 16 in PA is a crime because the younger person is deemed to be unable to give consent. There may be laws in either state that make the age difference relevant to whether the crime is charged as statutory rape or as some other sexual offense but rarely is it ever completely legal for someone over the age of consent to have sexual relations with a person under the age of consent.