I believe he has always been in existence since the dawn of time. But I do have a propose. Maybe he created himself overtime from a weaker version? That's just a speculation and we won't know until we all die.
Looter,
You are brilliant!! I really underestimated you. I kept thinking that you were a believer using irrational arguments to convert ex-believers. But now that I got the divine spark in my brain I realized that what you really are is a non-believer posing as a believer trying to show how silly, irrational and...well, giving our current knowledge of the universe, how ridiculous the argumentation of believers is. What you really want is current believers to feel embarrassed so they can research the subject a little deeper and inevitably become non-believers. Absolutely brilliant brother!!!