I'll just reiterate what the others have said... the disease must be under control, particularly any inflammation, and then, when all is well, you'll see growth. First weight and then height. What are your concerns exactly? Height? Weight? Lack of puberty?
My son is finally in remission and has has clean labs for seven months now. He started putting on the pounds very quickly once Remicade knocked everything into place. Now his weight is tapering off, but his height is really starting to shoot up. He's doing the classic "catch up" growth.
If you're concerned and your son's disease is under control, a visit to an endocrinologist might be in order. Although, at his age, you may want to go anyway just to check in.