CHEWY

why would a healthy n very smart 6 year old hit his head or punch his face when upset-mad?

Multiple reasons:

1) He observes individuals who hit themselves in the face when angry or upset and if parents or cartoons do it, then it must be the "common" thing to do.

2) Doesn't know how to handle anger. The anger or problem isn't from the toys or whatever the stimuli is but rather from himself. Or, it may be somehow better to hit onself than break a toy.