When you do ask about his day, ask about lunch and recess. IME, unlike class, the teachers say they are keeping an eye but they really can't during "free" time and kids' true natures come out, good and bad. All of my DS' tummy aches (and a couple of nuclear melt downs) have been because of recess incidents.
DS is in 6th grade and I still ask with whom and what he played at recess. It's a really good barometer of his social struggles at school. He is only now developing a vocabulary for the social and emotional currents among his peers but he could always describe events well if asked specifically to access the right memories of the day.
