N.B.
It's definitely possible that his anxiety is causing the hives. It sounds like something is going on in his little world that is causing him to feel overwhelmed. I have seen reactions like that in my sister's kids. My sisters daughter when through a similar phase when she was 3 or 4. I'm not sure if you are interested in alternative medicine, if you are I would recommend a naturopathic doctor, an herbalist or someone who practices homeopathic medicine. My sister found someone who uses flower essences and essential oil therapy and had a lot of success in helping her daughter.