Underweight toddler wont eat

I have a son who is almost 23 months and is refusing to eat. He only weighs 20 lbs and has been off the charts in weight since he was 6 months. He went from the 25th to below at 6 months. I have been giving him pediasure since he turned a year but I am just concerned. Any suggestions?

[Gina04]

Does he seem to be in pain or have abnormal bowel movements?  Pancreatitis or Cystic Fibrosis can cause stuff like this.  You might want to have him evaluated by a gastroenterologist.  They can do testing to check for food allergies or intolerances, Crohn's, celiac, etc. as well as checking pancreatic function to rule out things like pancreatitis and cf.  My son recently had to go through a number of those tests.