he doesnt need to think with languages
you dont need language to think
language was born becos of the thoughts you have
before language was born, you thought with your feelings
The language he learns to read? I guess the better question is born blind and deaf....in which case he'd probably think in Braille.
I actually quite like where this question leads.
Language is born from the need to communicate: you wouldn't need words for images and feelings if you had no one to express them to. I'll keep my answer short, so I'll say that the person 'thinks' in whatever language he has learned to communicate in. If he hasn't learned to communicate, then I'd say he doesn't think in a language, but is driven more by unconscious thought.
A big problem this question has that I don't have the time/energy to think about too much is that there is a distinction between 'thinking' and the 'inner voice' that the OP is gesturng at.