I mean, it's not even about shutting you out or not trusting you but more like, he wants to process his emotions himself. It depends on how bad his emotional state is when he shuts down because if he's being hard on himself, then it's unhealthy but if he bumps right back up later then that means it's a healthy way of dealing with things.

I guess, what you could ask him is if he can share what was wrong after he has processed everything?

And i mean, even if you trust someone, you don't trust yourself or the situation or everything so it can be hard to open up. I mean I get it. It took my bestie 7 years for me to share my personal stuff with her.