Send him an offer, for the amount you would like to be paid in order to complete the work that he has requested from you - in this case, the writing of an article. How much would you like to be paid, to complete this work for him? Send him an offer for that amount. Don’t begin any work until he has accepted, and paid for, your offer.
See previous answer.
You can’t, nobody on Fiverr can be sure of this. You can increase the likelihood of this by completing work to the best of your ability, whilst delivering great customer service.
Send an offer for the amount that you are happy with, to write an article. Forget about the ‘paid per click’ model, as other’s have said, it’s not feasible here on Fiverr. If I asked you to write me an article right now, what would it cost me? If it requires two hours of your time, what’s 2 hours of your time worth? (for example).
As other have said, this guy sounds like a scammer, and you’re best off avoiding. But, if you want to try, you could go down the route of sending them a fair offer for the cost of completing the work for them, nothing more. If they accept and pay for your offer, you get paid a fair amount to write an article. If they decline (which it sounds like they will), then you know you’re dealing with a scammer who’s trying to get you with the old ‘you work now, I (might) pay you later’ trick, which unfortunately is one of the oldest tricks in the book.