I make my kid smoothies- they are nice because you can make a big batch and freeze them. Whole plain or vanilla greek yogurt, frozen strawberries/blueberries/mango, banana, coconut water, other fruit you have on hand. I also sometimes put protein powder in it but if your son doesn’t like Boost the protein powder does have a similar vitamin-like taste. My son will also eat the Fiber One protein bars, the sugar isn’t out of control and they have a lot of protein and fiber. Good luck!