I'm confused.
Did he have an allergic reaction to the popcorn?
If so why did he receive antibiotics?
Unless he had a reaction AND he scratched himself raw AND he developed a secondary skin infection, then I see the point. Otherwise I remain confused.
Unless you do an elimination diet or have him undergo allergy testing (which isn't all it's cracked up to be), you won't know what he can have and can't have.
My girl is highly allergic to damn near everything, including herself, (her own yeast), but does really well if I keep her away from things I know she reacts to. Being allergic and reacting to the allergens are different things.
My girl can't have pineapple or honey. EVER. She breaks out in hives within minutes. She breaks out in hives for a number of other reasons but they are localized, not generalized.
She scratches due do inhalent and contact allergies.
Hives are easily treated it's the "other" symptoms that are hard to control, (itching, scratching, runny nose, etc....).
You have to find out HOW they are allergic and WHAT it is they are allergic to in order to make the situation livable.
My dog is allergic to cat dander but lives in harmony with her cat sister. I can't keep them apart and I can't rid the cat of her dander! But I do vacuum all the time and the cat has her hair brushed every day. It's not perfect but it makes a HUGE difference.
It took at minimum of a year to perfect our system. Some things are easily correctable, others are hit and miss.
If your dog can eat dog food fine then I would buy a food and have THAT be the treat. They don't know the difference, they just know they are getting something "special"!
Only give this food as a treat and the regular food for meals.