I can't say this is the answer
Well at least you are somewhat honest, let's see if it holds up.
but, the fact that he is an atheist means that he believes in God,
Not very long. How can atheism mean a person believes in god when the defining characteristic of atheism is the lack of belief in a god or gods?
because you cannot not believe in something that you do not believe exists.
That makes no sense whatsoever. What you are saying is akin to "I don't believe that the green and purple polka dotted unicorn exists, therefore I do believe the green and purple polka dotted unicorn exists." It makes no sense and cannot be logically resolved.
Perhaps if you where to ask him questions like, "why do you think we are here?"
Whether or not a person wants to think that there is some "reason" or "purpose" to life it doesn't mean that there is, nor does it make the existence of a deity any more likely.
or "where do you think we go when we leave here?"
Once again a persons belief or desire that there is an afterlife has no bearing on the fact that there exists no evidence of such. Hopes, beliefs, and desires for an afterlife do not make the existence of a deity any more likely given the lack of evidence of that "being's" existence. Just because a person believes they have millions of dollars doesn't mean that they do.
or "do you believe that you will have to pay consequences for your actions?"
That's nothing more than just another "I hope" situation, in this case a desire for infinite justice. And once again a desire of infinite justice doesn't have any bearing on whether or not such a thing exists.
perhaps those kinds of questions will trigger his imagination to be more open to at least speculate that there may be a higher being in this world
Or he could see through your fallacious "questions" just as I and my compatriots on FSTDT have. All of your questions have to do with a persons desire for such a thing to be true and have no bearing on whether or not such a concept exists. They have nothing to do with a search for truth and everything to do with "I hope it's true, I hope it's true". And none of them lead to the concept that a "higher being" exists.
and that people did not just involve on this planet for absolutely no reason.
Why does there have to be a reason? Why can't life it's on reason? Why does some "higher being" have to dictate an obscure and unknowable reason to life? And even if there were, which of the thousands of gods that humanity has worshipped throughout history set this "purpose" in place? Why did it do so? And how did you come across this special knowledge, and what makes you think that that reason and deity and not the others, are correct?
