The first thing is that you need to study Aqidah before trying to involve yourself in such questions or answering any non-Muslim/atheists questions.
Second, they will have as many questions more than you imagine, and these questions are asked in a wrong way, because their problem is not with Who created ALLAH or How the world existed..etc, their problem is with the ISLAM itself, and most of their questions are logically false.
Third, let him think what he think if he insists, we are now living in age of Fitna (temptation), and you are not a man of science or knowledgeable enough (nor me also), keep developing yourself and learn more about Islam, Worship ALLAH more by knowing and understanding his names, but choose your resource carefully, search a lot before judging something, and once more STUDY ISLAM MORE
Forth: If he is telling you that nature created everything around us, then you ask what is Nature to you ?
- The things around us, sea, mountains, skies, planets,...etc ?
- The laws that govern the movement of everything around us ?
- Some super-natural force beyond the this world and beyond our perception ?
In case 1:
this would be non-sense, because how is that the sky creates itself, or a sand creates itself? a normal human brain will refuse such thing. In addition, all nature things such as sky, land, stars, suns,..etc doesn't have brain, or eyes, or ears, so a low rank creature cannot create something more intelligent and aware-able more than it. What created something so precise as the planets movement in the sky without anyone crashing into the other, cannot be created by something that doesn't think.
In case 2:
here they are not answering the question and saying who created the universe, they simply stating how the universe works, not who created these laws that move and organize the universe, and make everything work the way it is.
In case 3:
If he say that there is a force that created the world that is super-natural, and that this force is very wise and powerful, and capable, and elegant and unique. We say it is correct, you call it super-unknown-force, and we call him "ALLAH". If he then say that he doesn't agree, then his problem is with ISLAM and ALLAH as a source of this universe.
In any case, this answer is for you before him, try not to go into deep conversations with such topic, until you become aware, and you should know that there are ways to prove the existence of ALLAH for Muslims and another ways for non-Muslims ... this is because Muslims already believe, but they might have a weak moment of suspect, so the way you speak with Muslim is different than non-Muslim.
May Allah always guide us to the straight path that will lead into Heaven.