14 mo Old Preemie Not Growing

I am going to cross post this several places, as I had it posted in a reply to another thread in Special Needs, Medical, and didn't get any suggestions. I hope maybe someone will have some experience or suggestions. Our ds (a through FC) was preemie at 28 wks and 2 lbs 3 oz at birth and drug exposed. He is now 14 months and 16 lbs, 6 ozs and 26 ins! He's now been surpassed in weight and height by our bio 6 month old. Basically he's stopped growing for the last four months. We have been struggling with "why is he not growing?" He is completely proportional, not skinny at all (even cherubically chubby). But we changed peds and got his medical records to take to the new Dr and found to our dismay he was diagnosed Failure to thrive (he never even told us!). Had reflux bad, and breathing issues (stopped breathing at 8 weeks and I had to perform CPR and call 911), hospitalized twice for lung issues (10 days each time), and has been on breathing treatments ever since the first episode. And is now recovering from another respitory infection. Only problems developmentally seem to be speech. We go to an endo next Tuesday and any suggestions as to what ?s to ask, etc., would be appreciated. We are at a loss, he looks nurished, the Gastro took one look at him and said he was proprotionate so no nutritional supp necessary. We'll love him no less if he is just meant to be short, we just want to make sure there is nothing wrong.