If you don't know which house the kid belongs in, the first thing I would do is to find out. Start knocking on doors and asking if it's their kid.
Now that you do know, the obvious thing to do is to introduce yourself to the neighbors and describe what their kid was doing. They're new to the neighborhood so you can pitch it as letting them know that there are a lot of passing cars at that point and kids should be aware of the danger.
A driver yelled at him, something fairly vulgar like "get out of the fucking road, asshole!"
He came home and reported this to me. I was more gentle obviously, and told him that the guy wasn't very nice for swearing at him.
But honestly, I don't blame the guy, and I'm happy he yelled at him. I'm sure he'll remember that incident next time he's cruising down the middle of the road.
If you do it by yourself. But you can always try to recruit another adult--a neighbor you know, for example. Having an independent witness makes a big difference.
Still not sure what I should have done if he hadn't gone home or really if this is a problem waiting to happen. At his age I wandered much further from home and put myself in worse potential danger at times, though I probably never yelled at passing cars. No one ever called the police on me though there were a few times when an adult told me to stop doing whatever I was up to or go somewhere else.