First, if he claims to be God and yet in fact is not, he has to be a madman or a lunatic.
Second, if he is neither God nor a lunatic, he has to be a liar, deceiving others by his lie.
Third, if he is neither of these, he must be God.
You can only choose one of the three possibilities.
If you do not believe that he is God, you have to consider him a madman.
If you cannot take him for either of the two, you have to take him for a liar.
There is no need for us to prove if Jesus of Nazareth is God or not. All we have to do is find out if He is a lunatic or a liar. If He is neither, He must be the Son of God.