There are many signs in ASL that are iconic, meaning that they look like
what they represent. For example, MOTORCYCLE looks like you're revving a motorcycle.

The miming that you see is actually a part of ASL discourse and is often used to describe specific actions. For example, if someone is discussing how people hunt, they might use the sign "HUNT"…but then demonstrate the throwing of a spear or the shooting of a bow or gun (or all three) to make explicit the type of…

Though the camera doesn't really focus on his signing enough to get all of it, from what I've seen, it looks like a mix of ASL and English-like signing (aka contact signing). His grandmother is Deaf. Given the context of the show, I think he's signing MOST of the content, but if some comments are lost, I can't imagine…