Help me find the name of this movie?

I don’t remember the name or any of the actors as I watched it as a kid, however I remember some of the plot.

The main character is a boy around aged 8-12, he may be autistic. He tells lots of lies and is very secretive. He lives with his mother in a house.

He is trying to solve a mystery of some kind,(I forget what,) and he goes to strangers’ houses alone to ask them questions about it, including: a very afraid man who won’t speak to the boy and has “yes” and “no” written on each of his hands, the boy lies to him and tells him he’s only four. The boy also visits a nice lady who makes him coffee.

The boy self harms by pinching his skin on his arms, chest and back, leaving red marks. He keeps clues to the mystery he’s trying to solve including pictures of people in boxes in his room.

Eventually, his mother goes through his stuff and finds all of his clues.

What is the name of this movie??