The human body has marvellous ways of ensuring that it is taken care of, which is why we need to respect it as much as possible. But having said that, each one of us has a different constitution so we have to be able to find what works for us.
The simplistic explanation of why we should not indulge in heavy physical activity after a heavy meal is that the body is busy with the digestive process and all the blood flow is directed to the stomach and digestive system. So the limbs do not get as much as the blood flow as would be required to help the body move about.
As with most young children who are busy thinking of when they can play next, your son finds himself playing after a meal, but does not seem to show any signs of discomfort. Therefore, although you can continue to tell him to slow it down after a meal and reserve his major game time till his stomach has settled (which could be about 1-2 hours later) if he is ok with the activity, then gently give into him a little! Or find alternate methods to keep him occupied that may not demand too much physical activity.