Here is an article that might help a little.
Don't believe a low number is significant but not sure. If there is an immune response they would be higher.
They (IVIG) are also given to patients (I had the treatment and it literally saved my life) with various auto immune diseases with very good results. Not everyone responds and there is some mystery about how flooding the body with antibodies works. One hyposthesis is that these are "good" antibodies and it helps to reset the immune system and stop it from attacking the body....but researchers are not sure.
Diagnosis of lupus and many other AI diseases is difficult and diagnosis is the result of labs and symtpoms. Here is a link with typical symptoms.
I would suggest going to a clinic like Mayos, Cleveland Clinic, Johns Hopkins (don't know where you live but there are a number of clinics associated with major teaching hospitals and universities)...where a team of docs can examine and test your son. Might not be lupus at all.
You could bring up the subject of your son to the doctor but most doctors are tightly scheduled and they would bill any treatment separately.
MCTD (lupus, scleroderma, polymyositis). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, Barretts esophagus, quadriplegic in 2005. Recovered and now active in skiing, tai, chi, hiking, golf. Meds: prednisone 2.5mg, imuran 50mg, amlodipine, benazapril, omeprazole, potassium, folic acid, vitamins, maxide and various supplements and vitamins.
Remain optimistic and you can overcome the odds.