The most efficient way is to show him results with obvious causal relation.
For example, if you wanted to demonstrate gravity, you would let fall an object in front of them and tell them this is gravity.
Mere words require a brain to process them and interpret them.
An act is more direct as you directly witness a causal effect so the interpretation part is less important than the processing (the witness moment) of it.
If the person acknowledges the truth during the processing, you insta-wins as there would be little or no debate of the interpretation.
I helps (a lot) if you can engage their intuitive sense(s) though that may have to wait depending on how aware or unaware they are.
A great teacher never strives to explain her vision; she simply invites you to stand beside her and see it for yourself. - Rev. R. Inman
To this end I'll typically make a psyball and put it in their hands and ask them to just notice how any changes in their being/input, typically at least 90% can sense it without training. Else ways live a magical life and they may come around, else not, it is not their time and you can do naught but wait.
tl;dr invoke their sense of wonder and curiosity