I can't think of a single time where you *WOULDN'T* use Dash's ability during the movement phase. So I would have your son ask his opponent if they are ok accepting that every movement will be with Dash's ability. He *will* declare during actions. Do that at the very start of the game before the first dial is revealed. ****, if you need to, have it as part of a setup checklist for him.
From there it just comes down to actions. When barrel rolling he has to get into the habit of saying if he is using Dash or not. That's where the real nuance comes into play. If he thinks a barrel roll will put him a rock, he can declare *not* to use Dash and try the move. If he hits the rock he doesn't have to complete the move.
This is the most graceful compromise I can think of and I would have zero issues as an opponent with it, regardless of age or skill level.
Edited by viedit