You do not state what type of testing will be done or your son's age. Over the summer, don't let his math and reading skills drop off. Encourage reading for fun, and math as part of games, etc. If he has a question on a word or concept, encourage him to look it up. Building models, playing with Legos (and other building toys), and putting together jigsaw puzzles encourage spatial skills.
I'm encouraging lots of fun stuff for your son over the summer that will secretly help his learning skills. When testing time arrives, make sure he is healthy and well-rested. No over-the-counter medication -- like antihistamines -- if possible. Remind him that he is already a good student, and being nervous or rushing through questions will not help him. Just encourage him to do his best. You can secretly be nervous for him. The purpose of ability testing is to assess a student as he really is. By helping him find learning fun over the summer, you will help him keep his skills sharp. Good luck.