You need to convince him to reframe it. Have you tried asking him calmly (when he's not gaming), "Hey, next time I play, how about encouragement?"

My immediate reaction would be not to tear you down, but build you up. "Ooooh! So close! You'll get it next time!" or "I saw it. Game totally cheated you out of that." I think this is something he should be practicing every time. It changes the dynamic and brings the focus back to where it belongs: on fun.