How can I help him?

My son was born 5.5 pounds, full term, IUGR, failed APGAR and was in NICU for a short time. Labeled Failure to Thrive...
LONG story short...  We were told he had RTA (reinal tublar acidosis) (please excuse spelling).
He has gone through endo, nephrology, metaboloic disease testing.  All we were ever told is that all his test come back normal but at the low end.

The question is...  How can I get my 8 year old, who weighs 41 lbs and is 4 foot 1 inch to gain weight.  He has many food aversion.. hates pasta, mashed potatoes, etc..  This could be due to the fact that a small part of his soft palate and uvula are split.  

Is there a supplement, or meds or something extra that we can give to him.  He is so thin. Sometimes he even loses strength in his legs when he runs.  (Sometimes I think he is losing mussel)

Please help..

[Renee Radenberg]

His weight is on the 3rd percentile and his height is on the 25th percentile for his age. He may appear thinner than he is due to being taller than his weight. Is he on any dense nutrient drinks such as Pediasure or milkshakes (cheaper then Pediasure) for extra protein and calories? Suggest in place of milk and at the end of each meal give him milkshakes (milk and ice cream blended together – add a banana if he will accept it) or yogurt smoothies (yogurt, milk, and frozen fruit blended together) or Pediasure. Add dry milk to oatmeal, gravies, creamed vegetables, and creamed soups (add additional meats to soups). Use all fat dairy products and no low fat foods so they have the calories they need. As they grow and gain weight after a few years, then you can go to lower fat dairy items. Hoped this helped you.