^sorry, I should have specified that he should look for social media groups for George Mason University.

I would have him search for a facebook or other social media site (maybe reddit?) where current students are listing their apartments for sublet over the summer.
My kids go to other colleges, and I see students/parents of students doing that all the time on the social media groups for my kids' schools.
Be very cautious about scams.

Any listserves for Fairfax? What about for George Mason area?

Friends of ours have a son who will intern this summer in Fairfax. He attends Hopkins at the moment. Any idea where he can post/search for available summer housing?