In the rheumatology world there is some flexibility in dose and frequency but this is in adult patients. Meds are often weight based in the pediatric population so this may not apply. I would talk to your son’s GI doc.
The onset of action for most biologics is fairly quick—within 1-2 months. This is of course variable between patients. Did he receive the loading doses? There can be continued improvement after 3 months but would expect to see some effect at the 3 month mark.
I’m sorry the Stelara doesn’t seem to be working. I hope you find something that does.