Get rid of the fire making and sharp objects. TSA and USSS / Capitol Police aren't going to like them.

Remind him texting often works when voice calls don't. (at 16 I am sure he knows)
Make SURE the cell carrier works where he is going.

Stay with the group. Help watch-out for less street wise members of the group.

Pocket hand/foot warmers (disposable air activated) DC area can be COLD when standing around.

COMMON SENSE, (maybe 6th sense). If it looks, walks, and quacks like a duck then it probably is a duck. Some ducks are friendly and some are mean, but they all want something from you.

Alternate plans to get home:

Recently a friends daughter had her group split by the airline. At the new airport, it became a two day SNAFU because the "group leader" then split that sub-group to try and get kids home on time for school. If it all goes bad, have the resources to go get your son available. I had to pay for these folks ticket to get her home. Did so gladly, but what if I hadn't been available.

Oh yeah, avoid Chicago O'Hare (ORD) airport at all costs. It just sucks.