What could cause my 4-year-old son to not have solid poop?

Might be juice: It might be as simple as juice. Most have high fructose corn syrup added and the amount of sugar is not healthy and can cause diarrhea. Much better to eat fruit! If this is a recent problem, it might be an infection, usually a virus. Has your child seen a doctor about this? If he is not growing well or having pain he should be checked for more serious concerns such as celiac disease.