Keep playing if he makes a mistake. Don't stop. That's a big no no at audition.
Stick to pieces he knows well. His grade 4 ones? He should probably know a little bit about the pieces too, but don't worry him this late if he doesn't. Make sure he knows the composer. They are unlikely to ask him questions - but might. Tell him to make an educated guess, or just say he doesn't know.
Maybe warn him that he might be asked to play some scales or sight read? It's unlikely though, and the school would have probably warned him prior to audition so, if he is a worrier, it is probably best not to tell him - too late to do anything about it now anyway.
Don't forget the sheet music for the accompanist. Taping in a strip is usual, and acceptable.
If he isn't a trained singer then to pick something he knows well, likes
and is within his range and suits his voice. I am sure your DS will give a good account of himself.
Tell him to embrace it, nothing will change if he doesn't get a scholarship so to relax and enjoy it.
Well you can try tell him that last bit.
Edited to add: Musicians never apologise for a mistake
someone thought they may
He's never to say sorry for any mistakes.