Praying Romeo will be okay and back to 100% in no time. You are a good mom for getting him to the ER and doing all of the tests.
Something similar happened to Max a couple of years ago:
http://www.yorkietalk.com/forums/sic...x-gagging.html
I thought he was choking. The ER vet couldn't find anything, but said Max's throat was red and irritated. The vet didn't think Max needed any tests, but took Max into the back supposedly to weigh him. I think maybe they did an xray as long as what they were gone.
We followed up with our regular vet the next day and Max was given an antibiotic. He recovered quickly and was back to full barking strength within 48 hours.
I still don't know what caused it. Max had a chew that day, but did not swallow any part of it (I was holding it). It still could have irritated his throat either from the coating on the chew, or scraping while he chewed.
The gagging came on very suddenly, and he was never out of my sight.