Water, cooking oil, food dye set, paintbrush (bigger is better), sturdy paper plates, paper (solid color)
Experiment by mixing colors and adding oil to see what happens. Then add one drop of food color onto the paper plate and have him brush the paper with the mixed oil/water base. The color of the drop should move gently around the paper with the oil/water mix gliding over the paper.
Cover the surface because the table can become messy and slippery with the oil.