Children with kidney disease usually develop a smaller stature due to the growth hormone and the kidney dysfunction. You can add calories by adding additional fat to his diet instead of protein if his acid reflux is not affected. Here is an example of a shake you can make: 6ozs Non-dairy whip cream, a drop of vanilla extract, and 1 teaspoon of sugar or 1 Splenda. Blend together. The shake provides approximately 400+ calories. Hoped that helped you.