Let him read the following dialog between God and a mortal considering determinism. It’s actually not very theistic, but merely presenting the free will problem in a logical manner.
It’s by logician Raymond Smullyan and it shows how untenable the position of extreme determinism is, without polarizing.
It’s one of the things everybody struggeling with the free will vs determinism should read.
What's the best response to someone who believes in hard determinism but also uses this to deny responsibility for any immoral actions they commit?
That is, they think all of their decisions were preordained, and then use this to claim that they can’t be held responsible for anything they do.