Indeed, Sir

okay, i actually have a theory even apart from the fact that the person who originally requested the picture asked for him to wear it: bertie didn't get him to wear it, he did it on his own, as one of his surprise expressions of cheeriness (i'm thinking like his NY romp in the television series). also, it endears him to small children, and bertie is probably somewhat disconcerted by it. maybe even someone tried to make bertie wear it and he refused.