So this behavior stems from something, but we don't know which. (Or at least I don't know which.
1. Worst case: He's a griefer and it's a passive-aggressive show of his disrespect for the stunt rule or the game in general or you. Penalizing his chance of success within the game just perpetuates the cycle of passive-aggression.
2. He'd like to come up with different stunts but his creativity fails him. Applying a penalty ups the pressure on a situation he just may not be able to fix on his own, and increased pressure isn't known for increasing
people's playful invention.
3. As a neurodiversity thing, he may have gotten "locked in" on that maneuver. Applying a penalty here isn't just likely to be counterproductive, it's genuinely disrespectful, possibly even anguishing to him.
4. Narrating stunts may not be what he's into, even though he likes the game as a whole just fine. That is, he may enjoy focusing on the plot, or "winning," or character development more than getting into a super-detailed blow-by-blow of this fight right here. Applying a penalty can "work" in this case, though it probably risks contaminating his outlook on the parts of the game he does like.
I would actually recommend a counter-intuitive second step to a productive approach. (The first step is the ever-popular "talk about it directly, human being to human being," offered up-thread. After that, if the behavior just doesn't go away . . .
Pay less attention to it. I don't just mean, "Try not to let it bother you." I mean, spend less game time on his stunt "choice." Not in a sneering, disrespectful way - that's paying more attention! It's also a dick move. Not in a way that reduces his character's effectiveness either. Just - here's your D8/D10, roll it
If the guy can't
do better at this aspect of the game, then it takes pressure off him (and everyone else). If he can't be bothered
to do better at this aspect of the game, then there's no reason for the rest of you to care more about his stunt choice than he does. Hand him his die and spend the extra time enjoying what the other players come up with.