My dad thinks Alexander the Great probably wasn't real, and uses the lack of original contemporary documentation of his life as evidence for this. I've tried pointing him towards documents like the Anabasis of Alexander or other works written by Greek scholars, but he says that all of these were forged during the Renaissance after being "translated" from Arabic. My question is whether the stone tablets or whatever ancient Greeks recorded their works on still exist, and if so where they're stored; do we have the physical remains of the original Anabasis of Alexander?
How does he explain the fall of the Persian Empire, the rise of the Successor Kingdoms, the spread of Greek in the East?
We have contemporary evidence for the rise of Philip in the speeches of Demosthenes, and apparently we have Philip's tomb. We have coins struck during Alexander's life bearing his name. We know the names of his generals and a good deal about the stuff they wrote, such as Ptolemy's account of Alexander's campaigns. The belief in Alexander was wide-spread enough that in the 70s, some 200 years after Alexander died, Sulla referred to Pompey as "the Great" as a sort of joke. Alexandria the city exists. For there not to be an Alexander would mean throwing out all of Classical History, or the assumption that someone faked his life very early.
What does his Alexander-free version of history look like?
he says that all of these were forged during the Renaissance after being "translated" from Arabic
With all respect to your father, he doesn't understand how Greek textual transmission works. The comment I linked specifically speaks of the Homeric tradition, but it's relevant to pretty much all the Greek texts that mention Alexander as well. It's a common misunderstanding, for reasons that I don't understand, that Arabic translations had an especially profound effect on our corpus of surviving works. Most of our texts are known independently of Arabic scholarship, and with only a handful of exceptions texts known from Arabic were preserved in the Byzantine east with traceable manuscript traditions. Besides, Greek texts are not the only texts from which we know about Alexander. Quintus Rufus and a slew of Latin texts mention Alexander at length. Unlike Greek Latin was never lost in the west...