As others have said — just be there. When he finally snaps and realizes how caring you are, he’ll be more forthcoming with his trust.
There’s also some great cognitive therapy “homework” online you can print and do with him. It really helps, especially the ones that force you to examine your thoughts from different perspectives.
They helped me a bit, and even more so when someone I care about participated with me.
Good luck and I wish you both well.