argument for God?

The argument could be stated as such:

p1. Only God could author/inspire a book with a lot of accurate predictions.
p2. The Koran/Bible/[any other holy book] has a lot of accurate predictions.
C. Therefore, God authored/inspired the mentioned book (i.e., God exists).

This is a valid argument. The problem is with the soundness (i.e., the premises are highly questionable). I wouldn't call this argument circular, however. I am sure you can find some excellent resources debunking this argument on Google.

"I do believe in God, because the Koran (he's Muslim) mentions a lot of scientific facts and predictions that we only knew about recently; and there's no way for anyone to have known these things back in 578 AD (approx)." - I would say it is an opinion

The main weakness of this is actually the claim that we 'only knew about the scientific facts recently'.  If they appear in the Koran, it strongly implies we knew about them before 'nowadays', indeed at the time that the Koran was written. 

The two claims are directly in contradiction, it cannot be in the Koran and also not be known about until nowadays because the Koran wasn't written nowadays and leaves a 'paper trail'.

You ask how to respond. I suggest the manner of Peter Boghassian. He has two books that should be of interest to you. One is “A Manual for Creating Atheists,” and the other is “How to Have Impossible Conversations.” 

Also, keep in mind the problems associated with cognitive dissonance. Festinger’s Question comes from his famous 1956 book, When Prophecy Fails. Suppose (1) an individual believes something with his whole heart and soul; (2) he has taken irrevocable actions because of it; and (3) he is then presented with evidence, unequivocal and undeniable evidence, evidence he himself fully accepts as true, that his first belief is wrong. Festinger’s question is: What will happen? 

Keep in mind, the most difficult beliefs for people to examine are those beliefs that have been (1) held for a long time; (2) adopted before the age of reason; and (3) most often repeated. Then add backfire, and you may want to give up the discussion.

It is all of these : weak argument, unsupported claim, and opinion. There's no circular reasoning involved, so no logical fallacies as such.

We have the same problem with the Bible - there are numerous predictions/prophecies which have failed, therefore could not be the "word" of an omniscient God.

Also, the Goddist fails to identify even a single scientific fact or prediction, so at that point some clarification is necessary, therefore the argument is really only half-made (weak).