It is all multiple choice and there is no penalty for guessing although if he doesn't finish enough I understand that the score report will reflect that his scores may not be accurate due to not enough questions answered. In hindsight, I think that this would be preferable to guessing wrong. Dd#2 took the WISC-IV at the end of 2nd grade with a GAI of 148. She took the CogAT in 3rd with a composite of around 118. I can't remember the exact the exact #, but it was about that.
The testing circumstances for her were less than ideal (people walking in & out of the room while the entire class was testing together, a teacher she didn't get along terribly well with, etc.). She did report to me that her teacher told them to just guess and not leave anything blank if they ran out of time which probably wasn't a good thing for her.
If he is testing on the 2nd grade or below form, it won't be tightly timed. From 3rd grade up, each section relies upon accuracy and speed. That was also probably not a good thing for my dd. With your ds' age, speed probably won't be as important.
