I use a lot of flaxseed. Omega-3's, lignins, fiber...all good. And cheap. I buy in bulk, organic, at the local health food store, (plug), Rainbow Bridge. I grind it in a blade-type coffee mill, and add it to bread, cookies, dog biscuits, in yoghurt with fruit, in hot cereal, on ice cream, and my favorite...(non-dairy) smoothies. I use a banana, 6-8 oz. either papaya nectar or pineapple juice, and ounce of flaxseed meal, an ounce of hempseed protein powder, 4 or 5 ice cubes and a tablespoon of raw, organic coconut oil. Blend until smooth. I put a lot of thought and some research into the smoothie recipe, and every ingredient is there for a specific reason. Tasty, and might work for your little boy. Probably halve the recipe for him, unless you are going to share.