23, male 5'5', 98lbs abnormal weight?

Hello,

I'm a 23 year old Caucasian male, 5'5 tall and only weigh 98lbs (doctor weight). I have regular blood check ups, had a thyroid ultrasound and doctors says I'm "healthy" and don't need an endocrinologist. But my height and weight is not proportional according to any medical growth height/weight charts. I'm a 23 year old stuck in a body of a 14 year old. According to charts, I should weigh at least 134-140lbs. I don't have medical conditions or diseases and generally feel fine, except fatigue.

Can someone give me advice if there is help for me? I'm a hard weight gainer and nutritionists only know about their weight loss diets for obese and giving common sense advice like eating healthy, lots of calorie intake, and lots of vegetables, fruits etc.. What medical specialist should I try to help with my underweight problem? What type of M.D.s deal with people with weight gaining problems? It seems like the medical community only focuses on treating obesity and not someone with my problem.

I live in NYC and if you know specialists please recommend. This issue is causing me a lot of problems with my social life and career potentials because I was rejected by the military for not meeting height:weight standards.

Thank You