Firstly, you will have to explain to him that as Muslims we have submitted ourselves to Allah, thus when Allah Ta’ala forbade marriages with disbelievers ,we have to obey, regardlesss of whether we understand it or not. This is the reason for us not being able to marry them. If you want to mention wisdoms behind it, then the greatest one is that the beliefs of the Kufaar classify them as being impure (Najis) in their beliefs. How can it be possible for a muslim to have as a soul-mate a person who denies the existence of Allah, our Creator and Sustainer? Belief in Allah is the very essence and pivot of the life of a Muslim, thus a relationship with a disbeliever is impossible for a true Muslim.
In addition to this, bringing up the children according to Islam would be difficult if not impossible, as they would be confused when it comes to religion as there will be a huge difference between the religions of their parents. There are many other harms as well.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai