I think the little soldier sprites from the original Command & Conquer are good for inspiration. They are only about 10 pixels high, but they have a bunch of different idle animations; doing push ups, smoking cigarettes, cleaning their weapons and so on. You could also just make him stand still and just tip his foot a little, as if he is impatiently waiting for something to happen. Normal breathing / chest heaving / shoulder rising is probably visible too much at this scale.