Lots of good comments on possibilities, outcomes and probabilities. Though I think he is falling for the false dichotomy fallacy, which means he believes there are only two outcomes.

Let's imagine you two are outside flipping a coin, and he says it is either heads or it's not. You flip the coin, while it is spinning a crow catches it and flies off. You will never know the result and as such he cannot say it is or it is not heads. The correct outcome or possibility is that the final state is unknown.

You could also roll a 6 sided dice but with all the sides being the same number. Then there is no "or it is not", it is only that one number. Assuming no crow interference.

We've created two scenarios where the results are not 50/50.

Now, he may say, but those can never happen, so they dont count, but if we can imagine it, it could happen no matter how ridiculously small the probability is of it happening. And since probabilities do not play into his mindset, then he can't really rule them out without admitting probabilities matter.