The questioner has already answered this '' My son's insurance company say he just has to accept that he is at fault because that's how it is if a car hits a pedestrian'' The same applies if a car hits a cyclist , it is always the car driver who has to pay out as cyclists and pedestrians do not need insurance. The only variable is the % the driver has to pay, the pedestrian may be held for example 90% to blame so the driver pays 10% of the total award.The award could be £100,000 so the driver will have to pay £10,000 ( or his insurance will) .
The driver will not be charged in this case but if the exact same thing had happened in daylight the driver would probably be charged with 'driving without due care' aas well as paying out. You are not free to hit pedestrians even at night on an unlit road , the fact that the pedestrian was hit means that a % of the blame must be with the driver . A 'perfect' driver does not hit pedestrians even at night on an unlit road.
By the way I nearly did the same thing, I was driving on a dark country road at night when I just picked out a black shape ahead. I slowed down, luckily, as it turned out to be a black horse with a rider right in the middle of the road at 3am . It was only because I was driving a Volvo with excellent headlights on full beam that I saw him at all, a lot of cars would not have had lights good enough to have spotted it.