My theory about “him” is that Grunkle Stan nor any of the characters who have “icons” surrounding the all-seeing-eye/Mr. Pyramid/”him” are not “him”. (This means Stan, Mabel, Dipper, Wendy, Robbie, Soos, Gideon, the llama person who might be Manly Dan, the writer of the books, or Sheriff Blubs, who are the symbols, are not “him”.) I believe this because the all-seeing-eye is “him”, meaning he is watching all of the surrounding symbols, aka people. So obviously, he isn’t one of them. He is a different character who is not included in the symbols. I think he watches them all so he knows everything and he has knowledge of who has the books, etc.
Also, this leads to my other theory about the writer of the books. I don’t think he is any of the other symbols, because the six-fingered hand has its own symbol, rather than being the same as one of the other symbols. So I don’t think any of the people represented in the symbols are the writer nor “him.” Thanks for reading!