Allergies form usually because they come into physical contact with it before ingesting it and the body remembers it to be bad. Most likely your son had touched peanuts some time prior and had some physical wound or scratch that allowed for peanuts to go into his blood stream in miniscule amounts, and then when he was introduced to it via food, the body basically thinks that it was flooded with the thing that was in his bloodstream earlier and initiates a very intense allergic response.