It's a subjective thing which can depend on the circumstances at the time because, technically, all vehicles parked on a public highway are causing an obstruction.
As regards road traffic legislationt, some of the relevant wording about road traffic accidents says as follows:
"If, owing to the presence of a vehicle on a road, an accident occurs......." Therefore, those words, i.e. "a vehicle on a road" could refer to your son's van which might then implicate him in the cause of the accident.
Hope this helps.