You are correct Virginia does have a slow down/move over law. In section 46.2-921.1 of the Code of Virginia it states that drivers must yield the right of way or reduce speed when approaching stationary emergency vehicles on the highway.
The driver of any motor vehicle, upon approaching a stationary emergency vehicle that is displaying a flashing, blinking, or alternating emergency light or lights (i) on a highway having at least four lanes, at least two of which are intended for traffic proceeding as the approaching vehicle, proceed with caution and, if reasonable, with due regard for safety and traffic conditions, yield the right-of-way by making a lane change into a lane not adjacent to that occupied by the stationary emergency vehicle or (ii) if changing lanes would be unreasonable or unsafe, proceed with due caution and maintain a safe speed for highway conditions.
Violation of any provision of this VA law constitutes a Class 1 misdemeanor. If the violation resulted in damage to property of another person, the court may, in addition, order the suspension of the driver's privilege to operate a motor vehicle for not more than one year. If the violation resulted in injury to another person, the court may, in addition to any other penalty imposed, order the suspension of the driver's privilege to operate a motor vehicle for not more than two years. If the violation resulted in the death of another person, the court may, in addition to any other penalty imposed, order the suspension of the driver's privilege to operate a motor vehicle for two years
In VA a Class 1 misdemeanor can come with penalties of confinement in jail for up to twelve months and a fine of up to $2,500, either or both. It is likely that your husband is required to attend on the court date due to the fact that jail time is a potential penalty. Other Class 1 misdemeanors in Virginia include: Reckless Driving, Driving While Suspended (Driver's License), and Driving Under the Influence.
This "move over" law has been in effect for 6 years in Virginia (since 2002) and at least 40 states other states require drivers approaching a police vehicle with flashing lights activated either to make an immediate lane change or to slow down at least 20 MPH under the posted speed limit.
From our research it appears there are currently 10 states that lack any form of a move over law. These states are California, Connecticut, Delaware, Hawaii, Louisiana, Maryland, Mississippi, Nebraska, New Jersey and Rhode Island. So while your home state does not have the move over law, most all other states do.
It is unlikely that your husband could get the ticket dismissed just because of being ignorant of the law. Even though it is difficult to do so, we as motorist are supposed to know the laws of other states that we travel into. Some states advise drivers of certain laws on signs as you enter the state, while many do not since there are too many numerous laws to put up signs for.
As for the move over law, it was likely publicized when it first came out but with most all states now having such a law the state of VA thinks that drivers should now know to move over or slow down automatically when they see emergency vehicles in the roadway or on the side of the road.
If your husband is convicted of this moving violation then the VA courts will inform the Maryland Motor Vehicle Administration (MVA). Since this offense is not a law in your home state of MD it is unlikely that it would go on your husband's Maryland driving record but he can check with the MD MVA to find out for certain.
Your husband should find out how much his ticket will cost by contacting the Virginia court listed on his ticket. The clerk of the court should be able to give him the range that a first ticket for this offense usually runs in the county he was cited in.