A little freaked out, we called the vet as soon as they opened to determine if this was something we should be worried about. Luckily, it did not warrant an emergency visit, but they did recommend coming in to speak with a doctor. After a quick visit, bloodwork, and a urinalysis, it was determined that he was otherwise healthy and there was no obvious reason for the tremor.
Our vet found some information on idiopathic seizures common in boxers, which manifest as head tremors. Most dogs, and also in Ed's case, are totally conscious during the seizure and can respond to commands. Most dogs (Edison, too), can be brought out of these seizures by focusing their attention on a treat (or in our case, us!)
We've not seen any sign of these seizures since, and as our vet said, may never see it again! We're just glad our guy is okay!