Any Advice - Sick of vets bills and feel like were going backwards. Dog has allergies to dust mites..?

We've gone through all the correct channels of Getting him tested for allergies etc, it started when he was 1 year old. Allergic to house/storage dust mites!! we have given him a few dosages of Steroids now this year, seems to be the worst year of all, he's only 4 ys old now. He's just come off a really strong dosage and straight away he's itching bad. I hoover and dust every day, use VIACUTIN tablets and have the INDOREX spray for his bed etc. I am running out of ideas now. Tomorrow I'm putting a cotton sheet on his bed to see if that helps. But lately I'm wondering if it's some other allergy, vets might be wrong! who knows. Anyone else have similar issue?