I think the testing is a good place to start. After I had the results, I'd ask for a meeting with the principal to discuss the results before the school year starts. It sounds like your son has a great deal of asynchronicity which will require a teacher who's comfortable with differentiation.

Will you be able to volunteer in the classroom? This can give you a lot of insight. You may also want to find math books that are the appropriate level and coordinate this with the teacher. At DD7's school, K was heavily weighed to the language arts and reading.