Favorite Color
A custom-painted wagon doesn't have to be an elaborate work of art. Let the little boy choose his own color for the wagon. Have him pick out his favorite solid color from a selection of paint choices. Whether he chooses neon green or pale yellow, chances are he'll take pride in his toy knowing he picked the color.
Camouflage
Little boys who have a family member serving in the military are likely very familiar with the camouflage uniform. Having a camouflage wagon may be ideal for a child of service members or any little boy who likes to play Army. You could also paint the wagon in a hunting camouflage pattern for young marksmen.
Race Car
If the little boy in your life has an extreme need for speed, reflect this in his wagon paint style. Turn the wagon into a depiction of his favorite race car, complete with numbers and sponsors painted on the sides. You could also take a more generic approach to the racing theme by adding flames along the sides or racing stripes for a classic look.
Character
Most young boys have a favorite cartoon or storybook character. Paint the wagon in a character theme. Make the base color something neutral or one associated with the show or story. In the center of the wagon, either paint a mural depicting a scene with the character or a giant picture of the character. Along the edges, include the name of the show, character or any logos associated with either.
Sports Team
Whether your little boy has already developed a love for a specific sports team or you're aiming to make him a fan, a custom-painted wagon can serve as way to show this off. Paint the wagon in team colors, complete with a logo in the center and team name down either side. You may also opt to include a painting of the team mascot as well.