You seem like you love and care for your friend so much. That is such a wonderful thing. Just let him know you care and are there for him.
Almost no one in my life is aware of my problems. You wonder why he has not confided in you. It is probably because he has shame and is afraid to share it. It has nothing to do with you.
You ask if you can trust him. I'm sure you can. I'm still a trustworthy friend even though I'm not comfortable sharing my problems with everyone.
It didn't sound like he's 'crazy'. It sounds like he's in a lot of pain and frustration. Her needs a friend that cares like you do.
If you saw that he has anxiety disorder, then read about it to get a better understanding of it.
There is nothing you can do but just continue to be his best friend. I'm glad he has you.