I have to say that I agree with other comments in that a limit is exactly that, a limit and not a target.
As far as I am aware there are no minimum speed limits enforceable in this country.
Different classes of vehicles have different maximum speed limits even in a 60mph zone, eg LGV and PCV limits are much lower.
Any accident that may be caused will be the fault of the impatient idiot at the back of a slower-moving vehicle and I think your son has at the very least reasonable grounds for an appeal.
Best wishes.