Same advice here - good amount of sleep, good breakfast, and a drink and snack if they give a break during testing.
Maybe get some analogy pages off the internet or there is a site called "Testing Mom" that will give you 100 free practice questions if you give them your email info.
We told both of our girls that it would be some fun riddles and puzzles to get them to really use their noggin. We told them to stop and think before they gave an answer and to try to answer everything even if they didn't know the answer for sure. We asked that they didn't give any "I don't knows". Our tester even mentioned in DD8's write up that she seemed to really think out each answer.
I would say testing for each girl was pretty high stakes for us and it was very stressful for me each time. I know how hard it is to not let that show. Try to keep things light and make sure your DS understands things will be fine no matter what. Give him a big hug (if he is a hug kind of kid) and smile and tell him to go shine!
