It's in the way that B.J. and Hawkeye leave the tent empty after a patient crashes under Charles' hands. It's in how they tone down the pranking when a difficult letter arrives from home.
In turn, it's him letting them wind up and go at their usual brand of humor after they've lost patients. It's him choosing to actually go to Rosie's on a night they will be at their most buffoonish, to lend solidarity in forgetting the hell that they live in.
Kindness, Charles has learned, is as much in the heart of the recipient as anything else.