I watched this video on YouTube about proving God's existence, and they said that it's impossible to make up a God. Like people have made up fairies - which are a mix of humans & winged animals. Most things that aren't made up have been thought up using other things. (I can't think of anymore examples.)
So if that's the case, who thought up having a God? What would a God be made up of? Surely if someone made up a God then it would have come out by now that God is real or not.

I'm not arguing, i'm just saying what you could casually mention to him without having an argument.
I don't like to argue about the existence of God. It's a matter of opinion on how you live your life. Some people accept it but others won't.

Just pray for him. Either to see sense or accept other people's beliefs.