Talk with him about his mother.

 

Reminisce all the situations/episodes when he was a child. His interactions/moments with his mother, the happy ones and the sad ones, the good ones and the bad ones.

Draw up two lists: 1) The things I feel responsible for, 2) The things others are responsible.

For each item on the first list, ask him: Do you have total control over it? If the answer is "No", move it to the second list and label it with the name of the person who has control.

One item on the first list should be "My own health."

 

(He may be feeling responsible (or guilt) for something (over which he has no control), and his mind is trying to run away by busying itself with anxiety.)

 

Disclaimer: not a professional advise.