If you want your son on your itinerary, book the one ways. If you book 4 round trips and 1 one way, he will be on his own itinerary and if there is a cancellation, there is a remote chance he could be placed on a different flight. Now, most of the time this happens way in advance and you have time to remedy it, but at least you would be on the same itinerary with the one ways, hence moved to the same flight....
Duds