Hives caused by a virus.

Well I have spent way too much time being sick and with sick people. My poor five year old has a rash all over his body that gets hives or welts on it, his skin gets really hot and itchy. It seems to lighten up while the benedryl is working but as soon as it starts to wear off it gets all inflamed again. The doctor thinks it is caused by a virus, which last week he had a lower stomach thing going on and has had a cough. I just want to know if any of you other mothers have seen anything like this? Any prayers would also be appreciated. Hope everyone has a great friday.