I am not sure if I can help but I can say my 9 year old daughter is the hive/rash queen. She gets bumps, hives, rashes every time we turn around. But never one that spread like you described. Now my neighbor has. They said it was a virus. They say with kids a rash is caused by either a drug or a bug. Bug, meaning a virus. I am sure you retraced his steps in the past week. What did he eat, what did he drink, what did he swim in, what did he play in, what did he do differently, ect. If he gets too itchy our biggest savor is that Aveeno Oatmeal bath powder you throw in their bathtub. That has saved us MANY times. Especially during chicken pox. I think I took stock in it last year. Very natural. But I always warn the mom’s with the infants and toddlers, your child WILL be slippery so add extra precautions when using this Aveeno bath stuff in the bath. Perhaps line the tub with a towel, and be careful when you pick them up out of the water. They can be as slippery as a jello square in a toddlers hand on a hot day. I hope he is feeling better soon!