assalamualykum akhi
This should be expected with some atheists, their problem is not a lack of proof of the existence of god, rather it is the fact that they are too arrogant to submit to god or something bad happened in their lives and they blame it on god, then they go on to say that god doesnt exist.
In order to refute his "alien theory", their are many prophecies in the Quran which cannot be explained by aliens, because of the fact that it is impossible to go back in time or to look into the future. Knowledge of the future is only with god, hence the Quran is from god.
Also his silly slavery theory doesn't match up with the message of the Quran. The Quran tells us not to be blind followers of society, rather we should be aware, deeply reflect, and think about what is going on around us.
also akhi I would suggest you prove to him the existence of god via logical deduction
The Universe has a beginning
Everything that has a beginning has a cause
Therefore the universe has a cause
The Universe could not have caused itself because saying it did would be tantamount to saying A mother gave birth to herself.
Therefore the only logical explanation would be that god created it.
god is eternal, he is the uncaused cause
surat al ikhlas goes very well with this argument