How can I ask him to stop doing this without sounding like a moaning misery?

Just tell him again that you love kissing him but that when he gets excited the pressure is too much against your braces. Perhaps he is a little embarred b/c he thinks he is doingsomething wrong - don't worry about this. Reure him, but don't allow this to continue because he acts hurt. He needs to communicate with you about what he thinks.