Is this an ADHD trait or just a general (unpleasant!) character trait?

Is 'clamming up' during stressful or difficult conversations linked with ADHD or is that just an individual character trait?

For a long, long time our 'conversations' have consisted of me talking and him supposedly listening. Even serious conversations about our ill daughter evokes the same response. Total silence. He will get off his chair and walk round the room or stretch or do something else physical but there is no verbal response. 

He emotionally detached from me a few months ago and is only living in the same house due to financial restrictions so I am at a loss to know if his response is due to his unwillingness to talk to me personally (in which case I would have no patience for someone who can't be civil or adult given the seriousness of the subject matter) or if it's a 'shut down' because of his ADHD.  If that was the case then I'd have more patience with him. 

Asking him directly offers no help. He just looks out of the window and wont engage.

Does anyone have any insight as to whether this may be ADHD related or not?

He's in denial about his ADHD and wont see anyone to get tested so it's not like I can ask a therapist or Doctor who knows him.