"Dating" is not a legally recognized relationship in NY and, as such, there is no law against a 14 year old and a 17 year old dating. If they have sex, it will be statutory rape.
The above answer is intended for informational purposes and is not legal advice. It does not constitute the creation of an attorney-client relationship.
If they have sex, the boyfriend can go to jail for a long time. If they stop short of sex, the boyfriend could still be charged with a lesser crime i.e. contributing to a delinquency of a minor, and still have legal consequences. This gets worse when he turns 18, and she's either 14 or 15.