Why won't my so gain weight?

My son is 22 months old and only weighs 21lbs. Last September he was diagnosed with EE(eosinophilic esaphagitis). A feeding tube was placed and he is on a very resticted diet. He is only able to have food that does not contain milk, eggs, wheat, soy, nuts, shellfish or beef. All the research i have done has showed that the treatment has worked and the child gained weight over time. I believe that there is something else going on. He has hypoglycemic episodes and his growth hormone level was low. When tests were done for the growth hormone all of them came back normal. When he vomits there is always undigested food from 4-8 hours prior, and its in his stool sometimes aswell. The dr's have run test after test and everything always comes back normal. Can somebody please tell me what is going on and why he doesn't gain any weight?