OK, my immediate panic is gone! The vet just called me back and she said that if he was going to have an allergic reaction it would happen right away (and furthermore it hardly happens in dogs).

She has prescriped some adult benadryl, so I'm off to get some -- he will get 2 right away, 2 at midnight, 2 in morning and 2 tomorrow night. It will help with the itching.

He's still acting pretty weird -- begging to be in his kennel, panting, and walking in circles.

Thank you so much for the prayers, we appreciate them. I'm headed to the store right away!!

Edit: Mark just found some, so now I can stay home and keep a close eye on him!