I would talk to your GI about this, and ask for a referral to an endocrinologist. They can tell you whether his bone age matches up to his real age, or if it is delayed. If it's delayed and the x-ray shows a bone age of 14 for example, then you may have some extra time and he could potentially gain height. However, if his growth plates are closed, then I'm not sure what they can do.
It is also useful to look at his weight and height percentile. For example, if a kiddo has been in the 70th percentile for height from when they were a toddler till they were 10 years old but then dropped to the 20th percentile at age 14, then that's concerning. But if a child (like my daughter) was always naturally petite - she was always around the 10th percentile for height and stayed at that, then less concerning. The same goes for weight.
Your son was diagnosed pretty recently - how is he doing? I'm glad he's gained weight - that's a good sign. Kids typically begin to gain weight and grow only once the inflammation is under control.