|I guess I'll respond to myself. I spoke to our doctor's office, and the nurse said it sounded like my son is having a severe allergic reaction to the poison ivy, causing the bumps on the rest of his body. She also said the spreading out of the original rash is pretty normal. She told me to use hydro cortisone on his rash, and give him Benedryl every 4 hours. This cream works a lot better than the Caladryl. She said he would need to be seen by the doctor if he got feverish or lethargic, but he seems to be doing fine.
I hope this can help some other panicky mom.