Alan,
Hopefully it would deflect the weight (obviously a straight and direct blow would increase the danger) or at least take the brunt of the force, hopefully giving enough warning/time to turn the head. Although the weight your talking about is more than we tend to use unless fishing a flooded river, 2oz is the most we need in general and that would be at a range to remove much of the danger we're talking about here.
Some protection is better than none, I'll keep making sure he wears them, he already knows the other benefits of them and asks for his pair when we go anyway.