Well I don't know of any relationship between Asthma and Celiac ., but that doesn't mean it can't exist. There are levels of allergens in different breeds of cats and dogs . The Siberian and Oriental shorthairs cats don't affect 75% of cat allergy sufferers and the Balinese, Javanese, Devon and Cornish Rexes and Sphinyx are also less allergen. Many Terriers and Poodles are also "hypo Allergenic". So his level of reaction could be breed based , maybe?
Also females have less of the allergen enzymes as well as neutered males vs unneutered males, light colored coats vs dark colored coats etc...just a thought !!
I only know all this as I am severely allergic to cats and lesser to dogs. My husband and daughter are CAT people and I am trying to find a breed to work for us. :-)
I have had to use a nebulizer myself when I had respiratory problems , It worked tremendously. good luck