If they haven’t already identified him by that point, they figure out where he came from and start combing through “Patriot” events in his area of origin.
He has got tons of identifying traits, he’s wearing fairly particular clothing items with patches in a certain configuration. These people tend to have their "event uniforms" and don’t change it up much.
He will be IDed. He probably has been already.
Edit: Upon another look, I missed the completely obvious on him. His Blue Lives Matter emblem is shaped like Tennessee. There we go.