Puffin - you could be on to something about DS not recognizing friends' feelings. I would say more al hiong the lines of not knowing how to respond, but not recognizing could be fair game too. I still supervise playdates pretty heavily. I try to stay far enough away that I don't interfere, but close enough that I can hear and gather information to coach afterward.
It is hard. If they are both shooting with their own ball it limits interaction is what I meant by parallel play. If they were just taking turns with the ball (not comoeting for it) you son may notice the frustration earlier. Still the other child's parent is there too and could step in and coach her kid. It is not your son's job to stop the other kid getting unreasonably upset but it might make more sense if he saw when it started. And maybe you could fix up a code first so if he needed help responding he could ask for a drink?
And finally he could have been unlucky enough to get 2 sulky, do it my way or I won't play kind of friends.
