My 3 year old son have white spot inside his lower eye lid (both eyes). have any idea what is that?

See pediatrician: If your pediatrician is not comfortable with his exam of your child's eyes, please see ophthalmologist--(eye doctor)

Hard to say: But since they appear to be symmetrical in both eyes, and you don't describe any acute illness, they may just be normal granulation tissue from the past. If there is redness, swelling, any discharge or pain, you should go to see your child's doctor for an examination and proper diagnosis. Take care:)