Do you know how much Thor weighs and the concentration of your Amoxi? If you really want to, we can help you figure out the dosage for the Amoxi if you have that info. For this purpose, the Amoxi would be just as good as Baytril. If the Amoxi is for someone else, or you don't have atleast 2 weeks worth of it, he'll be just fine until you're able to get him to the vet after you've gotten paid.
It honestly doesn't look that bad. It's a small cut, it's just the placement that's making it look so big. His shoulder and movement makes it open more, which makes it look alot worse. Even a glancing bite can make a pretty decent sized cut, their little teeth are so sharp. Just in case, though, check his cage and make sure there's nothing he could have gotten caught up on, or any sharp edges that could have done this.