Underweight kid

My son eats a healthy diet, full of whole grains and fresh fruits and vegetables.  He has always been thin, but in the last couple of years I've become concerned...he is rail thin, every bone in his body is visible.  I've started making him protein shakes, mixing dry milk into his regular milk and giving him supplemental drinks, like Boost, to give him more calories.  I don't want to give him foods high in fat or sugar, but I want to get extra calories in him to give him a bit more "padding".  We had his pediatrician check him over and he doesn't have allergies or any health concerns.  What else can I give him?  He isn't a picky eater.

[Renee Radenberg]

His weight is on the 5th percentile and his height is on the 25th percentile for his age. You are doing a pretty good job so far for extra calories. Here are some more tips for additional calories: If he likes pasta, then add cheese sauces to the pasta instead of plain tomato sauce or plain pasta. Add cheese to vegetables, if he eats them. Add dry milk to oatmeal, mashed potatoes, gravies, creamed vegetables, and creamed soups (add additional protein sources to soups such as meat). Add beans to rice instead plain rice for additional protein.  Suggest yogurt smoothies with fruit - yogurt, banana or strawberries (or any type fruit), frozen fruit blended together which has extra calories and protein.