As a Christian, he isn't going to accept any evidence you provide from the Koran. You would instead need to make an argument from the religious texts that he believes are valid.

To a Christian, the Koran is nothing more than a bunch of made up stories at best. It has no authority or authenticity to Christians.

You would instead need to try to make your case from the Bible. It's a hard case to make since the Bible pretty clearly indicates he is God, though it is always by implication rather than directly claiming "I am God." Rather it says Jesus made statements that, to the people at the time, would imply a claim of Godship. (And in several cases resulted in them trying to kill him for blasphemy. ) At that point it mostly degrades to trying to prove those claims miss attributed.

You are also unlikely to make much progress in convincing him as well. This debate has been going on for centuries with both sides thinking their view is abundantly supported and obvious.

The benefit of interfaith dialog is much more on making sure we understand each others views. Fundamentally you aren't going to convince someone logically one way or the other. You can counter some objections that result from misunderstandings and ensure your views are well and clearly understood, but that isn't going to convince someone to change their beliefs unless a misconception of your beliefs was their main barrier.

(Full disclaimer, I am a Christian answering this from a Christians perspective in the interest of encouraging interfaith dialog. )

In my opinion, it's the fact that having children is a quality not befitting a god. We as humans have limited capacity to understand what "god" is. Anything that we as humans consider "human nature" does not apply to god. While we use the same terms such as "seeing" and "hearing" for God (i.e. God sees all, God hears all) its not the same "seeing and hearing" as humans. Edit here: God does not need eyes to see and ears to hear. Keep in mind also that Islam doesn't believe that God created man in his image. Likewise, the concept of having children is something that God gave to humans. It is not something that befits God. If God can have children and humans can have children, then we share a quality. We share a trait. God is unique and having children by definition makes him not unique. God by definition has to be the one and only, with noone to share powers with. How can God have kids? If Jesus had children then God would be a grandfather? If those people had kids and so on and so forth then there would be a whole generation of people who are literally related to god and are God's themselves? God is one and only. He has no son, nor is he a son, he has no father, nor is he a father. For me this is the easiest quality to understand about God.