DD took EXPLORE as a young 8-year-old. It was a pretty long time for her to sit and I don't think that I would have had her take it much younger.
Can you possibly just have him take the WIAT? DS took that at age 6 and it really was not obnoxiously expensive (yes, DD's EXPLORE test at age 8 was cheaper). Age 6 does seem young for the EXPLORE (just in terms of needing to sit still through multiple sections). Also, I am guessing he'd have to qualify under the Grade 3 criteria for DYS, since that is the youngest age. I suppose you could submit it as a portfolio item, but that is not as clear cut as the WIAT score would be.
I will consider the age appropriate test for him. Thanks for your suggestion.