Compliment him. No matter what he says or how he acts about it, your boyfriend (like everyone else) appreciates hearing nice things about himself. We all seek validation for how we look and what we do, especially when it comes from someone we care about. It’s good for your boyfriend’s ego, his pride, and his happiness.
Don’t make this more complicated than it needs to be. If you like the way he laughs, tell him. If you like the way he styles his hair or treats strangers politely, tell him. If you can compliment him on something you know he really cares about, like his basketball skills or his math grades, all the better.