free conversation with you as his parents.

I love questions like: Can you help me/us understand. . .?

Or: Imagine if I were your son, and you were my parent, knowing what you know, how would you help me?

Or: Can we talk openly? Do you sincerely believe Mom and I love you? What do you wish we knew about why you are struggling so much?

None of this can be accusatory at all. It needs to be open and sincere, and ensuring him of how you are trying to help. Also, put him in the position of where you are, and he can’t use the answer of “I don’t know.”