It is true that religion is to keep people civilised. All actions if Islam - it is not because Allah needs us to do anything, he does not benefit from us. However, he has bestowed the best of knowledge to human on how to live and prepare for the afterlife - to benefit ourselves.
The Qur'an, is far from just a book that could be made by a philosopher. Tell him to name one other philosopher who was able to claim that he has the solutions to all Worldly problems, present
AND FUTURE? - People often forget Islam has the solution to all problems :|..
Could an illiterate man have such depth of knowledge that he naturally got this knowledge? It would not be logical to believe this. Truly he was given knowledge only that could be described as divine...
Humans make mistake. If your brother was to assume he was just some smart guy - lets clear 2 characteristic tests.
He claimed God exists and he is the messenger. If he was just a smart philosopher - It would mean he is either a liar or crazy - he could just claims that certain actions would benefit human kind and would have no reasons to make up a God to argue his philosophical case. The latter would not work simply because someone insane could not bring up such great depth of knowledge. The former could not be true either due to his perfect characteristic! If he was a liar, he would not be a good person - which is what he was - not just good but the perfect.
People at the time were extremely ignorant - a fool could never convince them to leave there way - the way in which one makes most of his time in dunyas luxuries. Why would they give up there women, wealth and ways due to a philospher? This has never happened before, present or will not happen in the future (talking about philosophers, not prophets/messengers).
If he was merely a smart philospher and you assumed he made up God so people took his ideas seriously to make people civilised - this also is wrong... he would not risk his life as the way our prophet :saw: had done so - or been protected in such divine ways as our prophet :saw: was in delivering the great message... It would have not been worth it. Claiming there god was wrong and there was another God would not help a philosopher at all - just danger him and have him mocked, attacked, torturued and threatened - Proven by what had happened to our prophet :saw:.
There is nothing to support the idea "He was just a philospher" - no one can claim this without ignorance.
My cousin gave this link - I think it would be a good read:
http://www.islamicity.com/science/theamazingquran.shtml