Oh my gosh, that sounds awful! :( Most of my experience is with goldfish, and just a bit of betta knowledge, but I wanted to at least reassure you that as terrible as it seems, even missing eyes can heal without complications. I have heard stories of goldfish having eyes sucked out by other fish and they live happy and healthy lives. This may sound strange, but I would actually head over to Kokos goldfish forum and post there because goldfish are particularly prone to that problem and they may have more experience with it. Bettas are solitary fish, so it is unlikely that many people here have had this problem. Do not worry too much yet, as terrible as it looks, the injury is not life-threatening. Worst case you may have to start hand-feeding him, or separate him for feeding times somehow so that he doesn't have to compete for food. If you pick up some plastic mesh from a craft store, and some plastic report binders, you can make a very cheap tank divider and just put it in during feedings. Here is a link for how to make a divider. Good luck and fins crossed for a speedy recovery for your little one.