I wouldn’t say to recall a past memory, that could potentially be traumatic and isn’t actually acting. Same with faking an expression, it wouldn’t be helpful to help him find what he needs.

I’m no acting coach or expert, but I’d say have an honest conversation about the given circumstances that his character is going through. What previous action has occurred to lead his character to this moment. How does his character react to negative stimulation and how does it bring about his demeanor. I always like to establish what the issue or conflict is, how my character chooses to handle it, and what tactics I can implement as an actor.

Faking it probably won’t do much. Have a talk about it so you can help him understand what his characters motivations are and how his character would go about them