Sulky Boy, a portrait of Daniel Wynkoop, Jr. when he was three years old, was painted around 1908 at Cloverdale, the Wynkoop family's rented plantation in Virginia, on the Rappahannock River. The boy's father, a doctor at New York's Bellevue Hospital, had befriended Luks while treating him for alcoholism. The artist visited the family at Cloverdale at least once in 1907-1908, at the time this painting was executed, in the hope of curing his illness.
While at Cloverdale, Luks was photographed with the young Daniel, who is wearing play clothes—called "rompers"—similar to those he wears in the portrait. The child is probably depicted in the dining room of the estate. The fact that Daniel was asked to pose in one of the most formal rooms of the house may explain his tense, reserved demeanor. Or perhaps he was pouting about having to come indoors and stand still for any length of time, which explains the artist's title for the painting.
Luks concentrated more on depicting the boy's mood than on the accurate representation of his surroundings. The floor slants upward far too dramatically while the folds in the tablecloth are painted with horizontal brushstrokes that break the natural, downward flow of the fabric. Even the boy's legs are portrayed in an unsteady pose. It seems probable that Luks painted these details quickly from memory or a photograph after having depicted at least the boy's face during a sitting. This quick execution shows Luks’ training as a newspaper illustrator, when he had to document his immediate impressions of a scene.
Painted not at the request of the family but on the decision of the artist, the portrait appealed to the critics both for its painterly qualities and the emotions it evoked. Duncan Phillips declared the work an "example of combined humor and kindliness, ...strong but not brutal....Luks is seldom in this quiet mood."