Maybe try not attending practice for a week or two.
He could just be trying too hard. Or he could be feeling your presence during practice, whereas during the game anything not on the field is "zoned out" and thus it isn't a problem for him.
That is not to say that you are doing anything to cause such, it could merely be your son putting pressure on himself.
Or this could be completely off base.
I have seen kids who didn't perform as well in practice as the game, because they were focused in the game but not in practice. But this does not sound like your situation. So it has to be something else.