Stelara does NOT help axial arthritis in axial SpA or AS.
So if his MRI showed SI joint inflammation, then it's not the best option. Untreated axial inflammation will over time lead to more erosions, more sclerosis and then finally ankylosis (fusion). Both my girls have fusing SI joints and have progressed from axial SpA to a full blown AS diagnosis (an AS diagnosis requires damage to SI joints that shows up on x-ray, not just MRI).
Based on what your profile says, it sounds like he so far has axial SpA (visible inflammation and slight damage on MRI, but none on x-rays). If he is HLA B27+ he has a higher chance of progressing to AS.
Regardless, Stelara does not work for axial SpA OR AS. Any spinal disease (including SI joints) and it failed in axial SpA trials.
And we were told those trials were done with 90 mg every 8 weeks - so not the regular Psoriatic dose (which is 45 mg every 12 weeks). But I'm not 100% sure about that.
Unfortunately, we ran into a similar situation, where my daughter's AS did not respond to anti-TNFs. So eventually we went with Cosentyx, but as MLP explained, it's an IL-17 inhibitor and actually made Crohn's worse in trials.
M has severe AS but mild Crohn's and we felt we had to take the risk of trying Cosentyx because her joints were just getting worse and worse, pain was unbearable, she wasn't able to sit, sleep, walk etc. At the time, her Crohn's was well controlled on Simponi, before she switched to Cosentyx (her Crohn's responds very well to anti-TNFs like Remicade, Humira but her AS most definitely does NOT).
So we tried it, and within 3 months, her Crohn's flared. Added Imuran and played with the dosing for a year, but it caused too many infections. So eventually we got a second biologic approved - Cimzia (which is an anti-TNF). The combination worked for her.
So since your son has moderate Crohn's at the very least, not sure Cosentyx is an option.
But in the US, we were able to 10 mg/kg of Remicade approved - every 4 weeks. I'm shocked your GI says that it's not commonly covered - we had no issue at all. We did 10 mg/kg every 4 weeks for quite a while. And then we did high dose Remicade - it has been studied for juvenile idiopathic arthritis, so we figured it was worth trying for my daughter's AS.
We first did 15 mg/kg every 4 weeks, then upped to 18 mg/kg and then 20 mg/kg. NO issues with insurance approval, but that's probably because M had tried and failed several other drugs (Humira, Enbrel, Remicade at a lower dose etc).
We are in the US, so can't help with names. But I feel like I have seen Canadian patients on this forum (both adult and pediatric) on higher doses of Remicade...Tesscorm's son was on a higher dose of Remicade (I think), so perhaps she can help.
Going to tag Pilgrim - I know Pilgrim's daughter is on a double dose of Humira (weekly vs. biweekly). She is also in Canada.