I have the same problem, only it started when he was about 7 months old. He was born at almost 9lbs, and stayed in the 75th percentile until a year old. He stopped gaining weight and within another year was in the 1% for boys his age. He is 5 and weighs 33lbs and is 3'2". He is so small. He went from being picky to flat out refusal to eat anything! He is seeing a G.I. specialist, and he may have some mild food allergies. Everything else has come back normal, upper G.I., tons of blood work and stool samples. We are going to see an allergist and from there it's the lower scope. We fear he has crohns disease. I am scared to death that he will get very sick and end up in the hospital from malnutrition. We have tried every trick in the book, all the Dr.'s suggestions, and nothing has worked. It took years of tests to convince his pediatrician to refer us to the G.I. specialist, but you may want to try. We have found out so much more in the few visits with her than years with a regular Dr.  Good luck, hope you find answers soon.