I agree with Humaworm-- Children's livers are still pretty much healthy yet from toxins.
Still, I'ts good to keep him off artificial sweetners, and pre-packaged foods with solvents, and even over the counter medicines ( a big liver damager) because who knows with this young generation's choice of chemical additives and drugs, all those immunization and flu shots, etc, how badly their livers are compared to 4 -year olds from past generations.
Also- if you have a family history of liver issues to-- I'd have him checked.