I'd like to say I'm a genius, but in reality, it's because I would puke my guts up after a weekly methotrexate dose and felt rotten for the next 5 days, just beginning to feel a bit human by day 6. (Psoriatic Arthritis, widespread Tendinosis, Psoriasis, Celiac, plus additional connective tissue crapness from EDS and lots of wonderful pains, random injuries and other systemic symptoms from that as well).
If he's on tablets, switching to injections is a lot less puke inducing, especially as there is no issues with absorption through the stomach, so a smaller dose can be more effective. He could still feel nauseous and tired, but an anti emetic that acts upon the brain rather than the stomach can help - and it is possible for the consultant to increase the number of days he takes folic acid from just one to anything up to 6 out of 7 (the 7th being MTX day). I also became completely unable to stomach milk, sweet or fatty foods around the same time - they made the nausea 100x worse - and stuff just tasted weird all the time, like tomatoes, fruit (and anything I couldn't cover in sea salt).
An im steroid shot can really help - I'd get those when I was starting to flare; the fact I was near to tears, totally antisocial and ready to tell everybody to bugger off and leave me alone to stay in bed when I'm generally tough as old boots and very active was the cue to request a nurse appt. It's not as well known as I think it should be, but my consultant says she's absolutely in agreement with the research indicating it isn't 'just (perfectly understandable) depression or being sad because you're in pain' because for so many people, it doesn't match levels of pain but disappears within a couple of hours of systemic steroids (it doesn't work with intra articular injections) or within a short time of the medication regime being changed as the inflammation reduces.
I switched to biologics, first Humira and then Cosentyx and the latter has been brilliant for two years now - no nausea at all - and I think a lot of people would agree I'm far more agreeable generally now I'm not exhausted, in pain and being a bit of a twat with marginally less patience than Caligula emotionally labile all the time. And I'm not dragging myself through the week with the sole aim of being able to do absolutely nothing at all for two days.
Other things that helped were a. Avoiding all oats and cows milk as well as the obvious gluten/wheat/barley/rye/frequently cross contaminated stuff from the supermarket b. Getting custom orthotics and wearing them in running shoes two sizes larger all the time and c. consistently exercising to strengthen the muscles that had weakened from the pain, inactivity and assorted injuries. Strength training, rowing and swimming have been the gamechangers for me since the biologics have made it possible to move.
Hopefully, you'll be able to get him reviewed and ask about other steps they can take if his active disease isn't being controlled enough. Because 'It's doing something, but it's just not doing enough' is a good reason for them to look at his treatment and consider progressing along the treatment pathways.
I can easily see how a smart kid could become more antisocial, less motivated to do anything and also have no patience at all with anybody else if he's dragging himself through the days, never feeling truly well. And on top of that, he's a teenager, too.