Malizma, as far as the play goes, have you explained that a stage play is make-believe, just like cartoons or movies are. They may seem to be real, but they are just pretend.
Here are a couple other thoughts you might pass on to the family of this boy, if they make sense.
On dad's next visit to the doctor, they could possibly take the boy along. We are most afraid of what we do not understand, kids and grown-ups alike. Perhaps if the doctor who treats his father could take a little time with the boy and his family to explain, in simple terms, what made his dad sick, and how they are treating it, it would help to reassure the child that dad is getting better.
A second idea may be to have the boy help with his dad's treatment. If dad needs to take medication at a certain time, he could remind his dad. If dad is supposed to rest at a certain time, the boy could help make him comfortable. The trick with all this, however, is to do it in such a way that the child does not feel responsible if dad were to have a relapse, or take longer to recover than expected. This child seems to worry more than he should already.
Good luck, I hope all goes well.
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Why would anyone ever remember that?