Hi Pene, your Uncle didn’t fear the presence and went so far as turn his back on him. Whatever caused the interaction, that energy presence tried to reach out to your Uncle.
Turned into what felt like a slap.
I don’t discount your Uncle’s story.
No, it’s not his imagination.
Those who are sensitive to energy sense altered states of being.
What comes to mind is the movie Ghost w/ Demi and Whoopi :)