My son has hives and I don't know what's causing it.. Please help

I've taken him to the doctor twice and the doc says it's not a food allergy but something is definitely triggering it and irritating his skin.. I'm so confused I don't know what it is. I haven't changed anything. same soap,same laundry soap, No new foods, nothing that I could think of. he's being his normal self. playing around, eating and drinking well..