A lot. It is a felony. The act of having sex with a minor is deemed to be illegal because minors cannot consent to sexual activity by law, much in the same way people are not old enough to buy cigarettes, enter into contracts or join the military before they are of an age of majority.
The State can prosecute any crime without any individual approving the case, or signing a complaint. Prosecutors represent the People of the State, not individuals like in a civil case.
Your parents cannot consent to a crime being committed, and in fact their position is irrelevant at least in the abstract sense that it does not dictate what the State would do in making their decision.
There are no exceptions to this law that i am aware of. Whether he went to jail or not would depend on many factors including his history and the actual charges, were any filed.
It is true that in many states, the age of sexual consent is 18 and that, in some of those states, a person 18 or older having sex with anyone under 18 is unlawful. However, that is not the law in WA where the age of sexual consent is generally 16 and persons under 16 can have sex with older persons if the age difference is less than what is specified in the statutes. The WA statutes are at http://apps.leg.wa.gov/rcw/default.aspx?cite=9A.44 .
In WA, a person under 18 is still legally not an adult and as such is subject to the control of her legal parents. The parents can get court orders prohibiting the boyfriend from contacting the daughter. If the boyfriend violates court orders, he likely will be in jail.
If there are pictures and videos of a sexual nature of the young woman, the boyfriend can be violating both federal and state laws against child pornography. Some prosecutors have even charged young girls who took pictures of themselves.
The age of sexual consent in states bordering WA is higher. Traveling across state lines may trigger violations of federal laws.