I believe he is not listed in the notes because he is a "DM's Choice" NPC. Basically he is somewhere around Red Larch, and can be found fairly easily by asking around. Not to mention the son would certainly know where his father lives (and could likely lead you to his own home).
I expect this was left out mostly as flavor text that a creative DM could add in.
Source: Princes of the Apocalypse, Chapter 6
The Believers area secret group made up of some of the most important people in town of Red Larch. Braelen tells you that Rotharr Hatherhand (his father), ... are the Believers that he knows.
So he's obviously a member of that group, and someone important in the town (I believe he is a town council member). With this information, he should be able to be found easily simply by asking townsfolk. Being a council member he is likely at the town hall, or moving around the town somewhere.
Note: It should be mentioned that based on other information in-game, the town council is not meeting currently. So there is always the chance they are in hiding/etc (e.g. not meant to be found).