It's not a great answer, but what about getting your family tested for allergies? My doctor did seperate skin tests for cats, dogs, and rabbits (which now I need to go back and ask if rabbits covered all rodent spit). I came up a 3++ on rabbits, 3 on cats, and 2 on dogs.
yet?
I guess I have been around too many unwashed dogs because my perception is that I am rather allergic to dogs (one of the reasons I don't have them), moderate to cats (and I have owned many) and my pet rats were never an issue. Perhaps it's the size of the animal contributes to how much dander of their favor they put out?
I am just thinking if your son turns out to be allergic to rodents, having them in his bedroom might not be wise. (Then again I had 2 rats in my bedroom w/o a problem that I noticed, but I am so allergic to dust I usually attribute my stuffyness to that.)
Vaccuming is the only think that seems to help my allergies.
Good luck with it!
-Z., owner of MANY pets
(and have to agree I am not into my fish nearly as much as the cats, rats or birds. Frogs are super entertaining.)