If you buy a ticket for your son just make sure it's for the BNA-DFW-BNA routing and not for your entire routing. If he's ticketed on the entire BNA-DFW-ONT-DFW-BNA and he doesn't fly the ONT leg the rest of his ticket will be cancelled and create a huge headache trying to rebook and reticket his return flights. If he travels as a lap child then I doubt you would have those problems, but might have trouble checking a bag to just DFW as your itinerary will be to ONT.