How much luggage does he have? Is he reasonably physically fit to handle the bags he brought?

If he checked a bag, it will go to LHR so he should follow that plan. In fact, the odds would be very slim of being rerouted at CDG onto an AF flight--if there are any--to LCY instead of LHR, especially with no status and given that AF does ground handling for DL at CDG. He apparently accepted the rebooking through JFK and CDG to LHR, which was his original destination.

IMO he should grow up and take the tube to his destination, which has apparently given him directions and is close to a station. A college kid should be able to figure out the best route with transfers. Many people in London speak English/American so he can ask for directions easily. Of course, a really pampered kid would take a black cab directly from the airport, especially if helicopter parents don't care about the money.

Does he have British money or a plan to get some at the airport upon arrival?