I live in Pakistan and a Muslim guy ask me Why christians says Isa(Jesus) is son of Allah?

I answer him 2 World's biggest religion's in the world, Lailaha elalaah Muslims believe Isa(Jesus) is not son of Allah, but mostly Christians believe he is, but where this idea come from ?because there's not a single verse in the whole can prove Isa(Jesus) is son of Allah, you can read with translation or without translation debate ends if anyone have any question feel free to ask thank you.