Congratulations on his pending offer. Short-term housing: it's best to find sublets, which aren't as cumbersome as apartments (that require you to pay 1st & last month's rent, security deposit, utilities, etc etc)--sublets are temporary rooms students typically rent out, when they go abroad or to an internship elsewhere.
If money is no issue, the university housing would be the best choice, as he could relate and talk to fellow summer students, and feel like he is in school. Let's not talk about the myriad of troubles he may face with a total stranger.
Pitfalls: No furnishings in some rooms (Dorm rooms guaranteed to give you a bed and a working bathroom), noisy neighbors, crime, hidden fees etc.
Pros: Cheap, and available.
To see a list of sublets for the Washington DC area, click here: http://washingtondc.craigslist.org/sub/