Soldato
He wasn't filtering, he was overtaking. Filtering is travelling 5-10mph faster than the surrounding traffic and that is on roads without junctions. You use extreme caution at junctions when you are just filtering, and at overtaking speeds as he found out you are travelling too fast to stop safely.
Agreed, filtering is usually 10 or so mph above the speed of traffic. In my opinion the driver will take 100% of the blame but in truth your step son should have been aware enough to avoid it.