Wow! Thanks for all the replies in such a short time. :) Let me take them from the top!
"do you have the windows open on the days he develops hives?" -yes, but they are open all over the house, and he doesn't have problems anywhere else "Do you have forced hot air heat or AC vents that might need to be cleaned?" - Nope, we have hot water baseboard, BUT, I'll have to take the covers off and clean in between there. "Did previous tenants have a pet?" Yup, they had dogs, but he was tested and is not allergic to any animals. "Washing walls and floors thoroughly might help." I'm going to do that this weekend. Should I just use warm water, or what kind of soap? "Any chance it was something he ate/lotions/bath soap/etc. he consumed/used on those days before going to bed?" Nope, nothing new at all. "Fleas? Bed bugs? Spiders?" My daughter does have an allergy to dust mites, so he might have developed it, too. I'll have to go buy special mattress and pillow covers this weekend. "Could the paint be new enough that it's giving something off?" It's about 8 months old, so that shouldn't be a problem any more. "Or could there be mold inside the walls in his room?" How would I know? There's nothing visible on the walls, but it's an old house (built in the 1920's), so it might be possible. He has gotten hives at odd times before, and the previous ones we tracked down to Boyscout brand microwave popcorn, and detergent. I'll scrub the room and get the anti-mites sheets this weekend and will keep you posted. Thanks again!
Britta | 
