Thank you for the question. While enzyme deficiencies are responsible for some digestive issues like lactose intolerance, I don t suspect that this would be the case for your grandson. If he has already seen a doctor who has done an endoscopy, I suspect that most if not all relevant metabolic/enzyme deficiencies have been tested. Eosinophilic esophagitis would be more indicative of a food allergy or intolerance which is likely why they have suggested such a restrictive diet. If he has not already been seen by an allergist, they might be able to narrow down specific food allergies. This could even explain some of the aversion to food that it sounds like he has had for most of his life. What I mean is, he may have noticed that certain foods upset his stomach or made him feel ill, but since he couldn t express it and wasn t really in control of what he ate, he became aversive to all foods. This does happen with some frequency in developmentally disabled children. I hope that the food restriction itself makes him feel better and he starts to gain weight again. As for why it happened in the first place, an allergist might be able to help.