Tell your son that if he can't see the clock before the exam starts, ask to be moved so he can see it!
Thanks Capers. We have established a routine of resetting our little clock, so that when the real test starts at, say, 13 minutes past the hour he doesn't get confused when he adds 45 minutes to 13 on the clock. He also knows that when he reaches quarter past he's one-third of the way through.
I can hear readers muttering that he should be able to work this out, but having had one child go through the test I know how nervous they can be on the day and want to keep everything very simple. Remember tests don't start neatly on the hour.
You probably know who I should ask to get a definite answer on the use of clocks. If so, please let me know !