That was one thing I emphasized with DS (especially since he was really REALLY young -- 7 1/2 -- when he first took it). I wanted him to be absolutely clear on the expectation that he wouldn't finish, that there would be questions he didn't know, and that there wasn't any minimum "good" score. Just taking it was impressive, and whatever he got was whatever he got -- I already know what he can do, so it's not a question of that.
DS just took the ACT for the first time this past weekend, and it was another round of the same idea. He didn't need to worry about score, or finishing, or anything - just do it, and when you get to something that looks too hard, guess and move on. What do you get if you do really well? donuts. What do you get if you get every single problem wrong? donuts. Yay for testing centers near Krispy Kreme (says she who is planning to schedule her own actuarial exams for the location right next to the Polish deli... Mmmmmm.... tests.....)

If you think he'll seriously freak out, then I'd skip it, but I do think it's a good thing to nudge them a little outside their comfort range (just a little!) so the second time is easier. At this point the Explore is a walk in the park - not because he gets them all right but because he knows it's no big deal. Standing in line with him at the ACT place I was struck by how freaked out some of the high school kids looked (and probably having a short 9 year old in their line didn't help... LOL)... I know it matters more to them, but it made me think that an earlier, laid-back start to it could go a long way to making it less overwhelming when it counts.