I have not seen a report saying he got hit in the head. I have only seen reference to shots to the body, and I don’t see any evidence of a hit to the head in the video.
The distance and clothing would have made a taser employment pretty difficult. Had the hit that took him down been a taser, then he should have moved after the Taser power cycle was over, 5 seconds after trigger release. I think he fell to the snow from a devastating gunshot wound, head, neck, or spine.