It sounds to me like there's nobody in this kid's life who has the presence of mind to sit down and talk to him about what's wrong.

The idea that nobody knows why he's angry is making me a little angry because I went through this as a child and eventually had to just manage myself by leaving home for hours on end. Getting lost in the woods until it was to dark to see, walking along roads for hours, or just going and sleeping by a stream until I felt like I could manage being ignored and/or yelled at for just being me and doing what made sense to me.

To this day, nobody cares what I'm going through. I manage myself and mask until I'm exhausted, them just keep going because I don't really have a choice. Nobody really cares that I'm struggling or hurting. It's just expected that I act 'normal' and I know that the stress I go through is shortening my life every day.

Talk to him, without anger. If you can't do it, find anyone who can. Understand what he's going through.

Anger is poison and it will kill him if it doesn't stop.