Why does he add film grain? Does anyone even like that?
Sorry for the late answer but I don't think he's adding in grain after rendering the animation.
Basically when you render using "cycles" in blender it'll make the entire image/animation look like it has a layer of noise/grain on top of it if your "sample" rate is too low.
This can usually be fixed by either raising the sample rate high enough to where it becomes unnoticeable, or use Blender's noise reduction. The noise reduction can tend to look a bit waxy though so most animators usually go with using a high sample rate, or a high sample rate PLUS noise reduction so the waxy-ness isn't noticeable.
That or they render using eevee which doesn't have any noise problems but doesn't look as realistic as cycles.
I'm sure if you got the blend files he provides on patreon you could fix this yourself. That or use one of those AI noise reduction filters but I don't think that would look as good.
TLDR: He's not adding it in post, it has to do with rendering the animation itself.