A has done IVIG (Privigen) for years. Almost all docs pre-treat with Tylenol and Benadryl. Some also use Prednisone or Solu-Cortef (A gets this). Fluids are usually given before and after. Because it's made from human plasma there is an increased risk for reactions, particularly if the individual is IgA deficient.
Vitals are checked frequently and the rate is usually kept very slow to minimize risk. A's infusions usually take 5-6 hours. It is important to hydrate well 2 days before and after infusing. This is to prevent aseptic meningitis. The most common side effects are headache, diarrhea and fatigue. Side effects are best treated with fluids and Benadryl.
A felt better immediately after her first infusion. I hope it works well for your son as well. Many kids switch to SubQ after a few months but A is a creature of habit. Just know that there are options.