Salam
...
I don't know what empirical evidence to offer except for the authenticity of ancient Quranic manuscripts. I can start with the Sana Quran. It has 2 layers of text, an upper layer and a lower layer. The lower layer has minor variants and is not exactly identical to today's Hafs Quran. Many people use this to discredit the Quran and claim it is corrupted. However, the differences between the lower text of the Sana Quran and today's Quran are simply spelling differences that don't change the meaning of the verses. The upper text of the Sana Quran is exactly identical to today's Hafs version.
This shows that the Quran as it exists today, was also in existence during Muhammad's time, and that the certified Hafs Quran has not been changed throughout history, aside from spelling. Plus, spelling doesn't count, because the Quran is a recitation and spelling does not affect recitation.
For more information, please see this link. Scroll down to about halfway and read from there.
http://ponderingislam.com/2015/02/05/understanding-the-sanaa-manuscript-find/Here is an excerpt:
"This will probably come as a surprise to some of you: some revisionist scholars have held that the Qur’an is some sort of composite text originating later than the traditional dating. The easiest way to disprove that something does not exist is to show that object existing! Since we actually do have a Qur’an dating from during this time, we can safely put to rest any fanciful ideas on the origins of the Qur’an."
This basically means that
the Quran existed in full form during Muhammad's time and that Muhammad's Quran was identical to ours except for minor spelling/numbering variants.
There are other ancient Quranic manuscripts such as the Tashkent Quran, which is currently, I believe, on display in a museum.
In the interest of academic honesty, it should be noted that early Qurans did not contain diacritical marks. That is because the Quran was revealed to Muhammad by oral transmission, and "honorable scribes" (described in the Quran) wrote it down. The addition of marks does not constitute corruption because these marks do not change the text; they simply make it more legible for people who don't understand Arabic. Anyone who knows Arabic can read the Quran without any diacritical marks, and Qurans without diacritical marks still, obviously, exist. However, marks have been added for clarity.
Sorry, I think I repeated myself a few times. You can tell me if anything was unclear. Hope you can convince your dad