With right lighting and food, he will grow some more. He may never be a really big dragon but they grow for a couple of years so he has time to do some catching up.
Not to be rude, but I remember you from when you had Nyroc and the others and think you never got the correct UVB lights for them? If they don't have the CORRECT UVB lighting, they will not grow as they should. His growth is probably stunted and he sounds very little for his age. Also, romaine lettuce or ANY kind of lettuce doesn't have any nutritional value for dragons. It's like eating water. They need collard greens, turnip greens, endives, etc. I know people have gone over your setups many times before to try to help you in the past.
A REPTISUN 10.0 UVB bulb is what he needs, or a MegaRay MVB bulb. Diet and lighting is what affects their growth the most. Good luck