Your son could also take Amtrak from Union Station, DC to Penn Station. Once at Penn Station, he'd take the LIRR to Great Neck on the Port Washington line~35 min. He would then take the N58 bus and that would drop him off right at the Academy gate. Typically, the bus leaves the station within 5 minutes of getting off the LIRR. You need to get off, go up the stairs, over the bridge and through the station to the curb to meet the bus. $2.50 in quarters or use the Metro card which you can get at Penn Station kiosk when you get the LIRR ticket. Metro card is good for up to 4 people.