We told our DS7 this summer when he took those assessments that he was going to be asked a whole lot of questions and he just needed to answer the best he could, and that they would keep getting harder and getting something wrong wasn't bad, it wasn't like a grade in school. He does not have test anxiety however, so the word test intrigues him and makes him more motivated ( he sure didn't get that from me, even today I get test anxiety...). We just explained that the tester was going to learn more about how his brain worked, and how we could better serve him in his school plans in the future from the information in the assessment. I only explained it briefly when I asked him if he would like to undergo the assessment however, and didn't mention it after that until we were flying to go to it. Even right before I just told him to do his best, and be confident. Over explaining would only add to possible anxiety I would think.
Good luck! I hope his testing goes well!
