We actually made an effort to be too casual about the IQ test because we did not want to convey our stress to our child. The tester thought that my child was delightful and helped her learn a lot about the Avengers and particularly Ironman
For us, it was a very high stakes test - but, my DS is not mature enough to understand that it was a "make or break" sitaution. And knowing that "he had to do well" without knowing other things might not only confuse him, but stress him too.
My advise is as above - given the age of your child, just let him know that it is a "fun activity" full of puzzles, games and memory quizzes. And we told our son that there would be a very nice person asking him questions to see how his mind worked.
It turns out that he had a great experience during the testing.
PS: This happened a year ago when DS was 5. If it were now, I have a stack of Critical Thinking Company books and I would pull out some pages on figural reasoning and abstract reasoning (where DS is not his strongest) and ask him to work on them a few days before the test. I might also get out our tangrams, pentonimoes and pattern blocks and let him play with those for a few days. Not exactly test prep, but it might get the mind focused on thinking about such things.
