My DS6 took the SB-V at age 4 years and 9 months. He did it in one sitting over 1.5-2hours. The tester made him take a quick snack/water break at the halfway point. My son really enjoyed theh testing process. It was a long day, especially with an almost 2 hour drive, but well worth it.
The acheivement testing was more of a struggle (WJ-III). He took that at 5 years 1 month and HATED the writing portions. He just didn't have enough fine motor to do those activites with ease. Our tester was great. They used a timer and broke it up into small pieces and surprisingly he ended up scoring pretty well on those sections.
Sounds like you found a great tester who is going to be able to do the appropriate tests. I hope you get some great data to help you plan for his educational needs.
