Chinchilla... Self mutilation?

For a while now we have been dealing with fur biting with one of our chinchillas. He would bite out parts of his fur, and he just looked patchy. We didn't figure much of it, because when we adopted them we got him, and his mom, and she has that kind of habit too, not as extreme, but she has like little fur biting patches too. Well about a month ago we noticed he had an open wound a little smaller than nickel on his stomach, we panicked, took him to the vet immediately, got him some anti-biotics, they told us to do whatever we could to stop him from chewing it. We tried to home make an e-collar, and he would not let us put it anywhere near him, if I got it behind his head or anywhere near his head he would flip out and start jumping everywhere he could away from my hands, he is usually very hand tame with me and my girlfriend. We gave him meds, he hated us for it, but it closed back up... After long shifts of watching him and alternating sleep trying to do our best to keep him occupied enough that he wouldn't chew it. Now we noticed that he's opened up roughly 2-3x the size it was before and is just chewing on it endlessly... We have been wrapping around his midsection with gauze and replacing it every 8-12 hours. We took him back to the vet and they just gave us more antibiotics and sent us on our way, but we cannot get him to stop chewing, the wound is larger, and are generally unsure what to do... We don't want him to just eat himself to death.. Any help at all is greatly appreciated...