Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern for Simon. It must be frustrating to see him so uncomfortable and not responding to medication.
As strange as it sounds, when I'm presented with a cat whose ears are itchy and yet there's no evidence of mites or infection, then I'd want to rule out a food allergy. I know it doesn't sound intuitive or logical that a the ingredients in a cat's food could trigger itchy ears but this is actually a fairly commonly seen problem in this species.
It's a difficult diagnosis to make although it's a relatively easy one to address: simply change the diet to one which is more hypoallergenic or one with a novel proteins. Options include: Z/D (from your vet), or Natural Ba***** *****mited Ingredient Diets, Nature's Variety Instincts line, Evo duck or venison, Nature's Variety Frozen Raw Medallions (I recommend that they be zapped in the microwave for 10-15 seconds on each side). Grain free may help in some cases, too.
I'll sometimes treat these patients with steroids although I might hesitate to do so in a cat this young. Over the counter options which may help reduce his scratching are anti-histamines with acceptable ones listed below:
Benadryl at a dose of 1/2 of a 25 mg tablet given twice daily or
Chlorpheniramine at a dose of 2-4 mg twice a day or
Claritin (Loratadine) at a dose of 2.5-5 mg/cat or
Zyrtec (cetirizine) 5 mg once or twice a day
It is important to ensure that the formulations used contain ONLY the antihistamine and are not combination products (e.g. Claritin-D contains pseudoephedrine, which could cause very significant adverse effects in a cat).
Sedation is a common side effect with these kinds of drugs.
I hope this helps. Deb