Help my itchy chihuahua!

(Nope, that's not a dirty euphemism!)

Twice a year - once at the onset of fall and again at the onset of spring - our chihuahua gets allergic dermatitis. Also known as "COOPER! STOP CHEWING YOUR BUTT!".

Our vet has instructed us to dose with benadryl or Zyrtec at the onset of symptoms. I've been using benadryl this round, as it's dirt cheap and we keep it for a sleep aid, but it only seems to be providing minimal, short term relief. I'm going to get some Zyrtec when I run to the store later.

But, for now, the poor guy needs some immediate relief. Over night, he's chewed the hair off of a 4" patch of skin. :( I distracted him and put some coconut oil on it - it helped, but then he licked it off. I also have some colloidal oatmeal bath, so I'll give him a soak in that to see if it helps or make a paste and apply directly to the hotspot.

Any other ideas of things I can do to ease his itching? He's absolutely miserable and things that worked last year (benadryl + coconut oil) aren't working so well this year. He was a stray, so I have no idea if he's inclined to a particular allergy but the vet blames it on unspecified seasonal allergies, because of the cyclic nature of his symptoms.

He's still eating normal, up and running around, playing, and such so the vet's office said it would likely be useless to bring him in. They would just give the same advice since he just has the itchies.