How do I set this boundary?

My parent's partner contacts me often (like everyday) just to chat casually. I find myself annoyed whenever he texts and I'm honestly not that comfortable talking to him. Since my parent is in the hospital right now though I need to be in contact with him to discuss serious matters relating to my parent.

How do I politely set the boundary that I only want to participate in conversations related to my parent's health and no casual conversations?

How do I set this boundary?
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