Chances are he's clinging to a behaviour that is safe, successful and familiar. He may need many repetitions of your words and your positive attitude before he can begin to trust you or to trust himself to experiment with new behaviours. Behaviours are not adopted in a few days. These develop over time and consequently will not be changed in a few days no matter how much attention you pay to the behaviour or attitude. People behave in a manner that's comfortable and successful for them.
Students tend to live up or down to their teachers' expectations.
If you see them as faulty people who need to be fixed up and straightened out, they'll supply you with plenty of work. If instead you choose to look for whatever is positive and build on that, they'll work equally hard to justify your faith in them.