Help with cat allergy

[AlwaysNapTime]

It's looking like my 9 year old son has developed a cat allergy. I also developed a cat allergy around the same age but luckily have outgrown it.
We have 3 cats in the house. One that sleeps on his bed during the day (have now stopped this and keep the door shut during the day) symptoms are:
-hives
-sneezing
-eyed blood shot, itchy, red raw and swollen
-congestion
So far I've gutted the room, removed all soft toys (cat has previously bedded down on them) washed everywhere down, planning to change the bed and hoovering mattress twice weekly instead of once/fortnightly, hoovering daily and I've just bought a lint remover for hair the hoover hasn't picked up. He also takes a antihistamine daily.
Have I missed anything? Is there anything else I can possibly do? Once I've been doing the above for a while will that help?

Petal wipes for the cat & a child safe antihistamine for him? Fwiw we thought dh was allergic to cats but after covering everything with dust mite proof covers and hoovering literally everything daily I think it’s dust mites.

The carpets going next, he only gets wheezy at home in carpeted rooms. He’s much less wheezy now and the dry cough that makes me want to murder him is much reduced.

He is totally fine through the day, it's at night when he goes to bed, also a carpeted room. I really, really hope it's not the cats 😩
I'll try the wipes, thank you.

Hand washing.
Ds is a bit stuffy with our dcats. Window open at night.

Window open, yep will try that, he's never cold so won't be a problem. Thank you.