I would talk to your pediatrician about his weight or you can ask your GI if you can see a dietician. Sometimes kids suddenly shoot up and their weight gain evens out. But sometimes they don't, and you don't want to see an overweight child become an overweight adult. It's easier to fix now (when you control what he eats) than to fix it later.
I say this with compassion because my own daughter gained a LOT of weight very suddenly, while on Prednisone. It was a real shock to us, especially since she had been naturally very thin her whole life (and had been underweight for the last 3-4 years, though she had gained and was at her "normal" weight when this happened).
She was not an unhealthy eater and certainly not a big eater. So it was not a calorie issue. Several endocrinologists laters, she was diagnosed with Cushing's syndrome, which can occur due to long-term steroid use. It left her with the puffy steroid face, dark purple stretch marks and an almost 30 lb weight gain.
Anyway, she has lost about half of it now. Even though she wasn't an unhealthy eater, she did have to change her diet. She did have to count calories and make a real effort to lose weight. And though the doctors said again and again that over time she would lose it all naturally (without dieting), she did not.
The puffy face took about 8 months to lose and now the stretch marks are fading. But 1.5 years later, I'm not sure she will ever get back to her old "normal" weight. However, she is a healthy weight and looks great. But it took a lot of work to get here!
All her doctors that we saw about this - multiple endocrinologists, rheumatologist and GI - encouraged exercise to lose the weight and she is trying to increase that but her AS makes that hard. But she can bike and swim, so she tries to do something every day.
I don't mean your kiddo has Cushing's Syndrome or anything like that (M was on and off steroids a LOT - about she was on them for about 6 months every year for 6 years and received many IV steroids and steroid injections in her joints because of the severe AS).
But he does have spondyloarthritis and extra weight will hurt his joints, over the long-term. There are even studies that show that anti-TNFs work less well on obese or overweight patients with spondyloarthritis or AS. And, as we learned, not every kid will automatically lose the weight that they gain with Prednisone (though many do - both my daughters always did prior to the Cushing's). Some kids have to work at it.
He may not even need a diet or anything like that - maybe just increasing his physical activity would help (and exercise is absolutely necessary with spondyloarthritis!!). And he is growing so it may naturally even out. But it's a good thing to get on top of early, so it's worth discussing with your doctors.