I would second Irena's advice. My DS6 is Dysgraphic, Vision issues, Anxiety and ADHD. MORE time available is always better. LOTS of breaks and snacks. Be sure to get some good protein in for all meals and any snacks you can.
For my child, he also needed MOTIVATION. He was "earning" for each subtest he completed and reminded of that with each. Both times we had a very flexible "child-oriented" tester who was aware and supportive of his challenges and allowing him to do his best work. At the end we went shopping and used his "test earnings" to set the budget for a new toy.
We were doing our last testing with an out of town specialist in gifted and 2e which affected the length of time we could use. Our tester helped us find a nearby (to the office) park where my son could blow off steam on breaks and we swam every night at the hotel in addition to using the gym. The tester was willing to stretch over as many days as we could and to change as the child needed for the length of the testing appointment.
Lastly, our tester "saved" subtests that required greater written output until the end of sessions.... If he began with anything that required a pencil, my son would massively shut down and not comply with subsequent tests.
I would certainly discuss your concerns with your tester and confirm that he/she would be supportive of providing some of these accommodations to allow your child to do his best.
