Hrm, I'd guess he's allergic to bird dander, not dust mites? So it's not the feathers he's allergic to, specifically.

I know that you can get tested for being allergic to birds, though I'm not sure it's on the normal slate of generic testing. (I had to ask for it because I was in close contact with cockatiels and wanted to rule them in/out; they didn't have that, though they did have parrot, and thankfully I wasn't allergic to that.)