It might help if you post which centre he is sitting at as specifics vary from place to place.
Dont depend on them being told when they are half way through etc - much better they track themselves.
While its good to give them a general idea of what to expect so also warm them that it could be slightly different so they don't get phased if it is.
Also make sure they understand that while it is a test the adults there want them to their best so will be helpful and understanding about practical things so they should ask if they have questions.
+1 for going to loo in break - some children don't like to in a strange place- and having drink and biscuit if offered ( even if they don't especially like it or you don't usually allow it) as it keeps them hydrated and energy levels going.
It's really important he is prepared for the large number of people who will be there and that you put on the calm face