l80

This. Actions are so important. It doesn't matter what you tell him or how many times, a huge part of helping him is showing him what you mean. I know that sounds cliche, sorry.

Showing him that you're going to keep it together shows him that he can trust you to do that. That's a huge step in building trust. Best of luck to both of you.

Do you see those outbursts as demonstrations of trust? Like, the more secure he feels, the more need he feels to test it, to make sure he's not being lulled into a false sense of security?

I can't tell you how brilliant that last bit is. Being a soundboard is ... fucking priceless. It's SO beneficial for him to be able to do that and not feel like you're going to judge or advise him. It also shows him that he can trust you and that you're a trustworthy source of information and opinions when you do give them or when he asks for them.