Hi A.,
First thing, have you been referred to a Gastroentirologist? They are specialists in these issues and are trained to test for a variety of disorders that may cause Failure to Thrive (FTT), when kids loose weight. You may also want to join a bulletin board or online group for parents with kids with failure to thrive. I learned a lot of valuable information from that. Is he constipated? Gassy? Have reflux? Any other symptoms that might inhibit eating?
My daughter, now 6, was Failure to Thrive from when she was 6 months old until 2 years. She wouldn't eat, she lost weight and at the worst, looked like a lolipop (big head and tall skinny body). She was also energetic and seemed fine otherwise,but wouldn't eat. She did have random bouts of vomiting that were always chalked up to be some stomach bug but always seemed so random and unaccompanied by other symptoms. She was also days away from a feeding tube when I learned from other parents about a medicine that is often prescribed for anorexics that encourages appetite. I had to persuade the Dr. but he finally prescribed it (it can be used for kids with FTT, but since that's not it's intended use, they are hesitant to prescribe it). Anyway, it saved us entirely from a feeding tube! However, it turned out that she had an underlying anatomical issue (a hole in her intestines) that was causing internal hernia's which would pop in and out, naturally making eating problematic. She finally had surgery to correct the problem and we've had absolutely no issues since.
While our case is a bit extreme, I do believe that while kids are picky, kids will not naturally starve themselves at this age, so I encourage you to reach out to some specialists that can determine the underlying cause...it could be an alergy or something else. But no matter how much pediasure you give, it won't resolve the underlying issue and you want to be sure you are at your absolute end before going with a feeding tube.
Good luck! And follow your Mama instincts....they saved my daughter!
~Liza