what is failure?

and my son is awarded a 'U' for his History A Level retake, after a year of not being at university, after £500 of private tuition fees, after hours of re-studying; this is all just too painful, especially as it's a lower mark than what he achieved last year.  How is this even possible?
I've had a shot at life and take 'failure' in my stride now, move on, but my son's frustration is much harder to tolerate.