Because Maryland and Virginia have a reciprocity agreement, you pay income tax only to the State that you *live* in, which I believe in your son's case is Virginia.
If your son lived in Virginia all year but worked in Maryland, he is only required to file a Virginia Tax Return. However, if his employer withheld any Maryland income taxes, he will need to file a Maryland Nonresident Form in order to receive the withholding back as a refund.