My cousins DD touched her sister's PB toast at about Oliver's age and immediately came out in hives and had facial swelling, she was then found to have a severe peanut allergy. I wouldn't think he would come out so much later if it was the PB.
I get hives allergies from brushing against some plants like grevillias or some antibiotics, they don't come up straight away.
Hope he is OK, its very worrying isn't .