Your three propositions does not make sense in french. We can understand what you want to say but it does not sound good. The last one is the most correct one.

A better translation would be (as proposed in the comment by @Alexandre d’Entraigues)

Je ne peux pas croire qu’il soit si grand !

The third sentence is ok, if you talk about something

But if you talk about someone, I would say :

Je ne peux pas croire à quel point il a grandi!