When DS was tested at the age of 5 we told him that we wanted him to take a few test to see when he can take special programs like the great astronomy class he just took in one of the museums. It wasn't exactly true, but it made him excited enough and guaranteed his cooperation
We made sure that we made it clear that there would be very easy questions, but that it was likely there would also be questions he wouldn't be able to answer. That it would be perfectly ok not to know a correct answer but that he should try to answer it anyway.
As for break, may be ask the psychologist to give him a break. My son took one for his IQ, but refused one for the achievement test. He was having too much fun, but I think a break would have done him good.
DS was also alone in the room. If it helps you can ask if you could walk your son in the room and spend a few minutes with him there.
Good luck
