Here is a not-so-good picture of the hives poor B was experiencing...

While we were at the allergist, B did such a good job! He was so brave, and didn't fidget at all while the nurses were poking him all over the place.
The diagnosis was that B is allergic to cats (which we have), dust (which we also have), and mold. The mold was his most severe allergy, and it confused us, because we couldn't figure out where there would be any mold in the house.
We finally found the culprit, hiding at the top of our vaulted ceiling.
This small area started as a teeny damp spot. We thought it was from an old leak. However, since we are now in the rainy season, that teeny spot has been getting bigger and bigger, and it is now looking to be growing some mold in it as well. This does not bode well for B, whose bedroom is on the other side of the leak on this wall, and his bed is up against it.
Looks like we are going to have to look into getting this one fixed sooner than we had anticipated! Good thing we're just renting! :o)