My 4 year old nephew also has lighter colored poop- but I'm wondering if it is because at that young age and size, they WONT have darker poops yet. Maybe their bile production is much smaller yet, so therefore much lighter brown bowel movements. this is just an uneducated guess from an ordinary person . I am not a doctor or nutritionist so I don't really know. I expect you will get much better responses from those who do!
How is his skin complexion and are the whites of his eyes dull? Is he low on energy? dull whites of the eyes, dull or yellowish complexion, Is he often constipated, or have low energy levels. These are some of the many symptoms something isnt' right in the liver.
If he does prove to have the beginning of a liver problem--I would keep him off of any sodas and only fresh squeezed or juiced fruit juices-- and the only pre-made juices I'd get would be Lakewood brand or Knudson organic-- or other organic juices that are not made from Concentrate -- just the pure fruit.
Allow him No corn syrup or high fructose corn syrup (so watch the labels on the foods he gets) and definitely keep him away from all artificial sweetners. Stevia or Agave nectar should be okay. I would keep away the honey yet as I heard it is dangerous for toddlers and babies.
Give him more raw apples in his diet- especiallly after a meal. My nephew loves apples!
If there is concern, Parasites also could be the culprit-- certain worms are very common in children but they often go undetected. I would check out Barefoot Herbalists' dewormer or Humaworm dewormer. Both are good safe herbal dewormers and both make them for young children. Also after your child completes a period of gentle deworming - You may want to give him a liver flush of sorts one Friday night without the Epsom salts if there is a concern for young children ingesting epsom salts.
-- having him eat only fruit and vegetable foods (especially apples) (no meat or dairy) up to 2:00 am, then only water with a pinch of sea salt throughout the day-- then right before bedtime give him a measure of olive oil with fresh squeezed grapefruit and some Black Walnut herb right before bed would be enough to flush out anything in his liver-- including any parasites or stones.
The dewormer will kill any parasites he may have picked up while playing, etc.-- and what I mentioned as a sort of ultra gentle liver flush will get the bile moving and push out any stray parasites (like fluke worm) from the liver -- mopping up strays from the parasite cleanse. Epsom Salts would make this more effective, but I hesitate to suggest having him use that part of the liver flush at his young age-- as well as that Epsom Salts in water tastes horrible in the first place (so i don't think a little child will accept it) At his age and size, I think a "psuedo" liver flush--following the protocol- wthout the Epsom Salts may still effectively flush out and clean his liver.
After all that cleansing is all over-- you may want to give him some herbal teas at night to help his liver heal-- Traditional Medicines teas makes a liver detox tea you can find in health food stores or buy online. It has licorice root and other herbs known to detox the liver.