I grew up in the Midwest and lived in the DC metro for two years so can relate to this. Personally speaking I couldn't wait to move back to the Midwest. But personal feelings aside, there are some things that can be done to really make up for the high COLA.
I lived just inside the beltway and was paying around 1100 a month for a 650 Sq FT studio. However, I was not close enough to the metro to use it consistently. I would advise a young professional, especially one in your son's situation where it sounds like he will be living with his girlfriend, to live close to the metro so they have that accessibility. It will help on commutes to the office, and also in getting to all the places around time. It will save time, stress, and energy. Arlington is a pretty nice area in NOVA that's close to a metro line, and I think you can find a nice place to live for a relatively moderate cost (for the area).
I might also recommend Fairfax but I don't think there's a metro line nearby, and I'm not sure of the COL.
One of my experiences with the DC-metro was that it was very hit or miss. You were either in a nice area, or a bad area. And there was really no inbetween, and you paid financially for living in the nice areas.
My parents live in Frederick, and personally speaking I think they've got the best of both worlds. Frederick is about 30 miles north of DC in Maryland. It's cheaper to live up there, but they don't have the amenities of DC itself. However, they are close to areas like Gaithersburg, Bethesda, and Germantown, which IMO have quite a bit to offer. Plus, there are enough companies and jobs outside of DC itself that you don't have to live in DC or close to it to make a good living.
If your son is a city person, he may not like the lifestyle in Frederick. It's more suburban, and further removed from the city. But IMO it's better than being in DC or close to it. You can still visit on weekends, but avoid most of the headaches of the beltway and the city itself. If I were to live in DC again I'd live in Fairfax or somewhere in MD like Germantown to avoid as much of the DC/Beltway traffic as possible.
So I guess I'd sum up this way:
North of the beltway in MD you have some nice cities - Bethesda, Germantown, Gaithersburg, and even up to Frederick that have something to offer, but you won't get much of what DC itself has to offer. Bethesda, however is close to a metro so that may be a good compromise.
NOVA I admittedly don't know as much about, but my sister lives there, and has lived in Arlington and Fairfax, liked both (she's a young professional, turns 26 this month). Arlington offers more of the city life and easy access to DC, Fairfax is nice as well.
Hopefully this offers some insight.