If he takes the inner loop of the capitol beltway (which is the fastest regardless of what mapquest says), he's covered with what you've given him. Interstate 95 in MD, from north to south, runs through Cecil County, Harford County, Baltimore County, Baltimore City, Baltimore County (again), Howard County, Prince Georges County, then exits MD into the Alexandria/Prince William area of Northern VA.
Cecil County:
155.5275 Fire Dispatch
39.24 MSP JFK Highway
Harford County:
460.600 Fire Dispatch (and 800mhz TRS)
39.24 MSP JFK Highway
Baltimore County (northeast side):
46.46 Fire Dispatch (and 800mhz TRS)
39.24 MSP JFK Highway
Baltimore City:
154.415 Fire Dispatch
154.145 Fire Response
154.385 Fireground
453.100 MdTA Police Fort McHenry & Harbor Tunnels and approach highways
Baltimore County (southwest side):
46.46 Fire Dispatch (and 800mhz TRS)
39.44 MSP Golden Ring
Howard County:
154.250 Fire Dispatch (and 800mhz TRS)
39.42 MSP Waterloo (also uses Howard Co. 800mhz TRS)
Prince Georges County:
495.0125 Fire Dispatch
494.8375 Fire/EMS Ops
494.6625 Fire.EMS Ops
39.36 MSP College Park