Our DS is a good test taker, I think, but will not be up to level on the English/Reading portions. I think we'd just tell him he really needs to show his stuff in Math, but for English/Reading just say relax/have fun/do your best/it doesn't really matter anyway.

I found testing dates that NUMATS uses.
http://www.ctd.northwestern.edu/numats/program/dates/
(I'm not sure if other organizations have different dates.)