White hairs could indicate a neck, brisket or underneath hit. However the neck would have dropped him on the spot via shock and the brisket more than likely would have left more dark hairs with the white(depends on the hide colouring he had, up here they tend to be greyish in the brisket area). More than likely you hit true and exited through the stomach area, given the angle, shot placement, finding of white hairs & what appeared to be stomach content this would be an acceptable theory. It could easily result in a poor blood trail as well.

Let us know how you make out and good luck with your bro this aft.