Hi, could have written your post just a few months ago. When my son started remicade in January he was 5'2" and 83 lbs, no signs of puberty, at 14.5 years old. 4 months later, no height yet but he is topping the scales at 100 lbs.
We felt like the puberty window was closing so we went to see an endocrinologist. She checked his growth hormone levels and review his hand xray. She told us that because of the malnutrition from the 1+ years of crohn's, he is about 1+ years behind. She gave us hope that now that he is gaining weight, the height and puberty will follow.
Hope our story helps, maybe a trip to the endo would help you and your son.