The Muslim would not agree with your assessment that he denies reason utterly. And, in fact, he doesn't. By using language or math, the Muslim demonstrates that reason holds. He simply cannot give a reason for reason. His presuppositions are inconsistent with his life and worldview. That is what presuppositional apologetics points out to him. But I think you are quite missing the point. We cannot begin with agreement on the existence of 'a God', simply because it is no agreement at all. The term must have content or communication is pointless. This is why all reasoning is circular, good reason being virtuously circular, bad reason being viciously so. How will you move from a shared term 'God' which,, as shared, has nothing of the Christian God in its denotation or connotation, --How will you move from this to the Christian God? You might as well begin with agreement that we both have a holy book. We wouldn't agree that theirs is a holy book at all, so there is no agreement at all. We aren't asking the Muslim to tweak his theology. We are asking him for wholesale repentance.