He can't stop. Like, literally can't stop.

Jacob watches as his father's mouth move, but realizes that something is wrong. David puts his hand over his mouth, but finds that his mouth is still moving. It was becoming abundantly clear that Alexa wouldn't stop until she was told to stop. And from David's standpoint, with Alexa currently using his mouth as a speaker, he would be incapable of doing so.

That left Jacob to simply utter the words, "David, stop"

The question is, does he?