Laura, I'm so sorry to hear that Corkscrew may have another URI. Please don't blame yourself. You're doing all the right things for him and are a great cat mom. Some cats are just more prone to getting sick than others. My RB Pepper was one of them.
Hopefully it's just allergies. Sometimes Sunny and Sky will get the sneezies and then it goes away on it's own.

Prayers and positive thoughts are on the way for Corkscrew. Please keep us updated and take care.