I am not a teacher.
Even in the first example you are talking about the (near) future, and 'You can't pass the exam.' doesn't mean that you think he will be unsuccessful. It could mean that, with proper intonation, but it really means that it's impossible for him to pass. You could use exactly the same sentence for the exam tomorrow.
To answer the actual question about 'can't' in the future, we would use 'be able to' and conjugate that. 'You won't be able to pass the exam tomorrow', or 'I don't think you'll be able to pass the exam tomorrow'.
In practice we would probably just say, 'I don't think you'll pass the exam tomorrow'.
Student or Learner