@Letskeepgoing oh the poor boy, he must be gutted. I once did something similar but turned two pages over at once and didn't realise til the last second. Multiple questions left out. I cried for days.
It seems trite but it isn't the end of the world. It seems like it is now, but even if it is a subject he wanted to do at a-level, he can get there in a slightly different way. I went to a community college at 16 and was the youngest person there by miles. Everyone else had taken a slightly different path to the expected one- teen pregnancy, refugees, even prison. Those folks were amazing and by the end of the 2 years every one had achieved what they set out to do. It might have been at a different time to their peers, but they did it.
Tell him from one person who did something similar to another- it hurts now, but one day it'll become a "you'll never guess what I did" anecdote, because in the long run it's not the grade that counts, it's how you respond when life trips you up.