What is the range given for bilirubin? On my daughter's blood work, it says Total bilirubin should be under 1.4 mg/dL. Online, it says the reference range is usually <1.2mg/dL. So if that's the case, your son's is more than double the upper limit at 3. And then his bilirubin in April was also high, if it was 1.8. If it was high both times, then I'd be less likely to think it's a random virus or a response to a vaccine (though I couldn't find evidence that the vaccine would increase his bilirubin), and more likely a reaction to a drug like Stelara. Is he just on Stelara or is he on other medications?
But if it's just the one time, then typically "wait and watch" is the approach doctors take. If it's really concerning, you would know. My daughter's AST, ALT, Alkaline Phosphatase, GGT and CRP were all so high last fall that she was hospitalized for further testing and a possible liver biopsy and we got a call from the doctor within hours of having blood work.
Typically, Bilirubin is done with AST, ALT and other liver tests - in a CMP (Comprehensive Metabolic Panel) or Hepatic Function Panel. I would get a copy of the blood work so you know what tests were done and how high they were.