Student told me he hates himself, what should I say?

I'm a martial arts instructor, and there is a 9-year-old boy in my class who's been a little down in the dumps lately. I've done my utmost to make these classes enjoyable, which seems to help, just not enough. He gets really annoyed when he makes mistakes, which is understandable, but then he told me that he HATES himself when he makes mistakes! He's one of my best students (apart from his current attitude). What should I say to him if he says something along those lines again?