Honestly, I would see a pediatric rheumatologist. Pitted nails could be a sign of psoriatic arthritis. BUT and this is a big BUT - most kids who are treated early and aggressively with JIA do well. My daughter was not treated early enough or aggressively enough and is also really an outlier - she simply has severe disease. That is partly genetic and partly treatment mistakes.
I would DEFINITELY see a pediatric rheumatologist before you switch biologics. While Stelara is approved for pediatric arthritis, it does not work as well for the joints as Humira and Remicade and other anti-TNFs and there really weren't studies done on kids with PsA - it was approved based on the fact that it's approved for adults (but if you ask any adult rheumatologist, they will tell you Stelara does not work well for the joints - probably because it is given at a MUCH lower dose for arthritis vs. Crohn's. I have heard of some kids responding well on the Crohn's dose though, while others got drastically worse). We really thought it would be a good option for my younger daughter given it works for both arthritis and Crohn's but have been told absolutely not, time and time again by doctors at HSS, Mount Sinai, Penn, NIH and another research study she's in.
Normally, if he does have psoriasis and psoriatic arthritis, they would add MTX and escalate his Humira dose to weekly before abandoning it. That would depend on the psoriasis though - if he has significant psoriasis and minor arthritis, then they may prefer Stelara. In my daughter's case, it's the other way around - very major arthritis and minor nail psoriasis and on her hand (not confirmed because we weren't able to biopsy the rash), so no Stelara. She tried another IL-23 inhibitor and got much, much worse so we abandoned it in 6 weeks.
How significant is his knee pain? Is his joint red or swollen or stiff? If not, it could be any number of issues and not JIA. Does he play sports?
It sounds more like he has skin issues and should be seeing a dermatologist. But just to be sure, if he does have signs of JIA (swelling, pain, stiffness particularly in the morning, a warm joint) then I'd see a rheumatologist.