Hi -
Yes,
under the supervision of a dietician,
INSIST on putting Zach on an elimination diet.
Also, my dd is 18 months and she is almost exactly the same size and weight as Zach. She is about 21.5 pounds and is 31 inches or something. Dh and I are petite, so we are not concerned about it and neither is our pediatrician. All we can do is offer healthy foods and she will eat what she wants.
If, as you say, Zach has always be in the lower percentiles for his weight, then it seems to me that he is naturally pre-disposed to being on the lower end in terms of weight. My pediatrician has told me time and time again that as long as a child is pretty much following their established weight curve, they are fine. And she also has said many times that children stop gaining weight at such a high rate once they enter toddlerhood (not a word really but you understand what I am saying!!
). So as long as there is not a DRAMATIC decrease in their weight, then my pediatrician says there is nothing to worry about.
I hope I am not coming across as pushy or a know-it-all, I certainly don't know everything and I am just trying to help. I feel like your doctors are not clearly explaining things to you and it makes me angry and frustrated. Your son's poos do not sound normal at all, and neither does his bad gas. I would arm yourself with the knowledge you have gained here about allergies/intolerances, and simply take charge of his care. Like I said before, call the dietician, and get him on an elimination diet. That would include getting him off of the pediasure since it does contain milk proteins. You may very well find that he will eat a lot more once he is no longer having gastro-intestinal problems.
Sorry you are going through this. Hugs to you and Zach.