Anonymous wrote:We're foreigners (now green card applicants too!) and it's not a double immigration he has to do - just the long line, with the grumpy officers who hate you and automatically think you're out to defraud the United States

My kids slept on the floor when they were little, in the line. We're treated like cattle.
If he's unlucky, he or any other foreigner (regardless of whether they're on a visa waiver, visa or green card application) will be asked to wait in a second area to answer more questions. My EU parents on a visa waiver once had to do that, while I waited for 4 hours at Arrivals not knowing what they were up to. But that's rare.
I would consult Waze for the most accurate driving time estimate, then plan to arrive an hour after the plane lands. At that time of the day, a lot of international planes land at Dulles, so there's practically no chance he'll be waiting for you. -and if he does, no big deal, he's safely arrived. But it might cut short your own waiting time, if his immigration process takes longer. I would never tell my college kid to take Metro after landing at Dulles. I would brave any amount of traffic. Bring a book and a travel mug of tea, or something

When I was waiting for my college kid at Dulles, I bought him balloons, ha! He's a dual citizen, but the airline lost his luggage, so even though he had no immigration process, it still took a while for him to make it out!
(I'd worry more about Border Control taking issue with his travel authorization, to be honest. The emergency number. That risk decreases massively if he's already traveled on his travel authorization without issue.)