I agree with Attorney Weber. Because you clarified that the age gap is less than three years, the crime (because it is considered criminal in Virginia) would be at least a class 1 misdemeanor, punishable by a fine up to $2,500 AND up to one year in jail. If she tells the prosecutor it was non-consensual, the consequences of course would be much more dire (felony rape, prison time). Please stress to your son the consequences of having a criminal record so early in his life.
The age gap makes the possibility of a claim by the girlfriend or her parents a very serious consideration. Illegality of sexual relations is not limited to non-consentual sex but can be based on the age of the victim in relation to the offender. It sounds like he will be over three years older than his "girlfriend" and the potential consequence could be devastating. Certainly at the very least contributing to the delinquency of a minor if nothing else. Relationships like these may start out as positive but inevitably if something happens, your son could face the potential for serious ramifications. Three year difference at this age is a big deal. In the twenties thirties or fourties, obviously that age gap is not such a problem.