El Camino, the Breaking Bad movie, is set to pick up shortly after the events of the series finale, which is why Brock Cantillo isn't returning for the film. Written and directed by Vince Gilligan, El Camino will continue Jesse Pinkman's story after Breaking Bad, and will hit Netflix on October 11, 2019.
A number of Breaking Bad characters are set to return for El Camino, with Jesse (Aaron Paul) being joined by Skinny Pete & Badger (Charles Baker & Matt Jones, respectively) on his new journey. There are also rumors of appearances from a number of other major figures from Jesse's life, including, of course, Walter White (Bryan Cranston). One person who hasn't been rumored to appear, though, is Brock, despite his close connection to Jesse.
The son of Jesse's late girlfriend, Andrea, Brock shared a special bond with Jesse, but it's believed he was sent to live with his grandmother after Andrea was killed. Brock was played in Breaking Bad by Ian Posada, and since the Breaking Bad movie comes so close to the finale, it'd be impossible for him to reprise his role. Brock was 8-years-old in the show, and that was six years ago. While it's plausible for other actors to return despite having aged a little, it isn't as drastic a change as it would be for Posada, who'd be unable to play Brock as a young child.
While Gilligan could theoretically have recast the role for the Breaking Bad movie, it's unlikely that it would have the same impact. The character's part would likely only be brief anyway, so he'd need to be instantly recognisable to fans. Replacing Brock's actor with another would lose that, rendering his role in El Camino somewhat redundant. It is a shame that Brock won't be able to appear, because he and Jesse had a great relationship, and the idea of Jesse getting a happy ending and raising Brock as his own son is a heartwarming thought.
Still, it is likely that Brock will be mentioned in the Breaking Bad movie. In the trailer for El Camino, there was a photograph of Andrea and Brock that was quickly shown, which suggests that they're still very much in the thoughts of Jesse. It's possible that he may want to go looking for Brock, but given that this is moments after the Breaking Bad finale, with the police very much after him, then it'd cause Brock even more trouble. Likewise, if Jesse is going to try to move away and start a new life, he might want Brock to go with him, but there are dangers in that too. That allows for tangible in-universe reasons as to why Brock doesn't appear, when really it's because of the setting of the Breaking Bad movie.