Coinneach's reply is actually pretty good. If you still aren't sure, interrogate the kid: "Who is this man?" As you approach (swiftly, swiftly... every advantage should be yours...), it would be well to get a description of the truck and its LP #.
It's important to begin the contact before the kid is put in the vehicle.
It's important to speak to the child.
It's important to get close enough before beginning the contact. Lots of stuff can be bluffed out from 50 feet away.
Remember that if the man gets himself and the boy in the truck, you're too late.