If you have to go beyond the white line after you stop at the stop sign are you considered in the on-coming traffic?

I asked this because my son was hit by a car from the on-coming traffic after moving beyond the white line and then was ticketed for it. The officer refused to provide an explanation. The weather was snowy, icy, and dark. He stopped at the sign and before the white line. He proceeded forward towards the main intersection, but never leaving the road he was on, to see past the obstructions to get a clear view of the on-coming traffic before turning left into the traffic. As he moved forward, a car traveling from his left, saw him, slammed on their breaks slid right into the front end of his car. As I mentioned earlier he was not in the flow traffic but on the road he was intending to turn from. Our only guess why he is at fault is because he was considered in the main intersection once past the white line. Is this true?