If cat dander is a trigger the thing to do is to have no more cats.
Asthma attacks are no fun (personal experience), although I've not yet
been bothered by cat dander, and haven't had a bout of wheezing in
years.  The best thing is to avoid triggers as much as possible.

He needs to find a good allergist/asthma doctor who can work with him and find
out what does and doesn't trigger the problem. Ask a vet or the local humane
society for recommendations. They may know someone that has successfully
treated other pet owners.
Going from Phoenix to Oklahoma is going to cause all kinds of reactions to a
lot of things. Very different climate and plants.
Could also be a reaction to mold or mildew in a new home environment.

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Unless the doctor tested him and he showed a positive reaction to cats,
there is nothing to say he is actually allergic to them. Bloody doctors get
me raging mad when they blithely tell a pet owner to get rid of their pet
even though the poor animal may not be actually causing the problem.I used
to be severely asthmatic. Even life threatening on occasions, and I have
always had lots of animals around me. My asthma was not animal related
though. I was allergic to dust mite and some pollens. I simply made sure my
home was as dust free as I could make it, and used antihistamies in summer.
Before you get rid of the cats, go to an allergist and get tested first.
Even if he is mildly allergic to cats, he will develop a tolerance to his
own 2 in time as long as he makes sure to get used to them gradually.

I had a boyfriend who was highly allergic to cats. This worked perfectly with
no medication. It will also get rid of other allergies as well.

No rugs or carpeting, only hardwood floors, tile or linoleum. No drapes, fabric
furniture, only blinds, shutters, leather and wood furniture. Easy to wipe down
often to reduce dander and dust. You do the wiping and vaccuming for her.
You'll save time cleaning also.

You wash the cat once a month to reduce dander. Every day you wipe the cat with
a moist cloth to remove hair and dander. Kitty loves it.

Get an air purifier and keep the windows closed so dust, dander, pollen can't
come inside. Wipe surfaces down frequently for the same reason.

Don't ever let the cat in the bedroom. Don't let the cat sit where he sits all
the time such as the couch or her desk chair. Don't leave her sweater lying
around for them to sit on. Tell her to rinse her hands after touching the cat.

Check your house for other allergens. Is a sink leaking and you have mold and
mildew? I heard of a guy who euthanized his cat as he SWORE it was the cat. He
still had allergies...and extreme guilt. I'm allergic to cats also and this
works for me too. Just keep in mind he'll still have allergies outside of the
house as other people have cats and you can't stop dust and pollen.