So sorry your son is enduring such a nightmare. Things that came to mind with so little info are infectious bites from insects( ticks or spiders), snake or rodents. Or an injury from playing or perhaps falling off playground equipment?

Depending on where you live and how much time your son spends outside are factors in insect and animal bites. However, I would assume by now if it were bacterial, your son would have a fever and they could tell if he had an infection and would give him antibiotics, not immunoglobulin therapy.
Maybe that therapy takes time to work and is not effective immediately?

Prayers for you at this stressful time.
D.