Timing is everything, as the old saying goes. "Make time for a casual exchange when both of you aren't preoccupied with something else."
Try to get him alone. If he's surrounded by his or your friends , he might respond based on their presence, and not based on how he actually feels. Don't worry if you can't catch him alone, though — just quietly ask if he has a minute to chat privately. Don't rush. You don't want to be nervous or hurried. Asking him as you're trying to speed walk or run to class, or he's on his way to an doctor or dentist appointment, is a recipe for disaster. Instead, choose a time when you have some flexibility, like after school or at lunch. Pay attention to his mood. If he seems particularly grumpy or quiet, it might be best to wait for another chance.