I used to work in one called Barnaby's many years ago but it is out in the suburbs - Wheaton - of Maryland...
The only one I can remember the name of in DC is Gallaghers - think it is somewhere near the Capitol.
Do you know where he will be staying? Agree Georgetown in general may be his best bet. Dupont Circle is also fairly lively if he is staying anywhere near there.
He should try to get adopted by someone local over the weekend and tag along to whatever they are doing!