I'd discontinue the Temaril. When new medication and new behaviors happen at the same time...I think they are connected.
That is way too scary for me as well. Call the vet that greytmonty suggested and get a second opinion too, and ask about other topical products plus antihistamines for dogs-there has to be one that you haven't tried. Poor Alan needs some relief.
From what you have described with his itching now, I'm assuming it is a dermatitis type reaction-so I'd start with a process of elimination for everything that currently touches him.
I'd start with laundry and wash everything you can that Alan lays on without soap, (run the inside of his dog beds through the dryer without a softener sheet incase it is dust mites or something left in his bed) then rinse the heck out of him even if it is just with a spray bottle and a towel. You could put some tea-tree oil in the water to help with skin irritation as well.
Where is he itching? Legs, ears belly bum? Where is he chewing? Any new foods? Any new travels? Long grass? It seems kind of silly, but if you can go back to when this started and provide as many details as possible (even though they may seem irrelevant) it would help us with allergy experience come up with some better ideas for you.
Hogan was even allergic to the fillers that they put IN the pills, so you never really know what the trigger is until you work through and eliminate all kinds of things.