I wouldn’t expect a refund. It’s your son’s responsibility. Ask him for the money.
It’s a lesson for your lad to not crash through better judgement and skill improvement.
I’ve been through this with ours – learning that crashing at the race means you don’t complete, or damaging a bike while practising the day before preventing you from competing, or crashing the first day of a long summer holiday after exams is not the thing to do, just so you can ‘go big’.
We’ve had a few wasted race entries, or burnt through a tank of fuel to ride for less than an hour etc. Now eldest is paying his own entries, own bike maintenance costs, he is riding is much more considered, smoother and faster.