It does sound like an allergy , have you used a differant cream, gel , anything new on him, its summer is he hot, it could indeed be
food so make a note of what he is eating, does the
rash Itch ? by swollen I think you mean like welts, they are called Hives and there are many kinds.As he is so young I think a visit to the doctor is in order he may need a cortisone cream try Calamine lotion .to help soother it ...