Four Eyed Opossum Tail Mutilation

I have several adult breeding Four eyed opossums. They are all in seperate large cages. The very first male I had was an adult and while he was a cage with a female his tail started to look red and bloody and he died within 3 or 4 days of being with the female. At the time of his death his tail was basically bitten severely and very bloody. I chalked it up to fighting and that she probably killed him. ( I also breed the STO's so i have seen this happen with them)

I got another male about 2 weeks later and put him with a female for about a week, his tail started to get bloody and looked like it was getting bit so i took him out. He died about a week after I took him away from the female (who ended up getting pregnant and had one baby as a result)

SO, I have yet another male and I put him with a female for several days and kept a very close eye on his tail, as soon as I saw any bite or blood I removed him which was about 5 days later. Heres my problem...He has been seperated from her for more than one month and his tail is VERY raw, bloody, and swollen all the time. I use blue kote almost every other day on it as ordered by my vet. It looks like he is biting his own tail, and it always looks very fresh and clean, like he cleans it constantly.

Has anyone ever seen this, or know anything I can do to help the little guy and prevent this prolonged injury from happening again?