Kingscote I completely get where you are coming from! DS fell onto tarmac whilst playing football at lunchtime a few days before the mocks. The hospital thought he had broken his scaphoid and immobilised his right hand. (It turned out just to a sprain). The school's response was to suggest that he writes and types left handed (despite having dysgraphia which makes doing it right handed hard enough!).
I spent a week really regretting that DS chose to play football that day. He made it through anyway, but did the exams either left handed or by removing the brace for the exam. Even a month later when the mock results were given out, no one remembered the conditions under which DS did them. Knowing how a broken wrist is not a solvable problem, I can totally see your dilemma.
However, it sounds like an amazing opportunity! Maybe someone else isn't available because of their exams, and so this gives your DS a chance to show what he can do?! If he doesn't take the chance, it might be something that he always regrets, and he is more like to emerge unscathed and feeling good about himself than end up injured, so, if I were you, I'd let him play.