It may not be control, but the president can influence it making it socially acceptable to say Merry Christmas if one so chooses by doing it publicly and frequently himself.
He can’t make people say what he wants. No President has the power to do that. Our founding fathers went to great lengths to make sure no president has the power to do that.
He can, however, work to remove the stigma around Christmas created by runaway PC. Then stores won’t be so afraid to say Christmas.