I think this is a good example of a lie because you and your son both have a shared understanding of the world and he is deliberately saying something he does not believe to be true (that does not match his own internal recall of events).
There is a physical component to this lie but it seems to me that the social part dominates.
That’s not the point. You know your son knows what happened - knowledge of his mental state. Not that you have a physical proof about the chain of causality.
There is a physical component to this lie but it seems to me that the social part dominates.