I know we say this all the time, but I think this is a time when a vet visit is in order. The issue with doggie allergies is similar to allergies in humans. Finding the specific cause can be very difficult and costly. What your vet will likely do is prescribe symptomatically. That's about all that can be done unless you choose to pay for the course of allergy test ($0,000s)
Our GSD has allergies that manifest in the winter when the house is dry. The vet told us we can give her human antihistamines. At her weight, she can take adult dosages. When she gets really bad itchiness, we take her to the vet and get a course of steroids. The vet has explained the risks/benefits. In our case, they come down better on the benefit side.
I can't say if any of this applies to Quincy, but I hope your vet will help you guys learn what's needed.
By the way, he is a handsome smoosh face. 
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