If the amount in box 17 of the MD W-2 doesn't match the other copy of the W-2 it would appear there's additional withholding not being reported. I'd suggest having your son check his W-2 to verify the state tax withholding.
All of his income would be taxable to Maryland, not just the amount shown on the MD W-2. If he paid taxes to DC, he would need to request a refund.
