If he's pretty comfortable with standardized tests (he's taken MAP before, right?), he'll probably have a good experience.

https://www.nwea.org/content/uploads/2014/07/ACT_LinkingStudy_1-17-2012.pdf

DD's MAP test scores were actually part of what encouraged me to have DD (then a youngish 3rd-grader) take the EXPLORE test (other than that, it was the wonderful folks here!). As you probably know, the key difference is that EXPLORE is tightly timed (as opposed to MAP). She didn't finish all of the sections (and chose not to guess, despite being told beforehand that she should - crazy), but scored well enough for the achievement portion of DYS.

DD didn't prep outside of the one sample test the Talent Search sends.

It was a great experience for us. If DD feels like doing it again this year, she might take it again. Please feel free to ask us any questions if we can be of any help.