Mating wounds do not look like bites. The bad ones look like large long slices.
As for why this is happening it is probably something to do with the make-up of your family unit. As your youngest mature and get older the older ones feel more insecure as the dominant gliders in the pack and things like this can happen.
You should have this boy absolutely ALONE because his sister could do a lot of damage to his wound by trying to clean it for him. this is probably how the small wound became a bigger wound so quickly. If you've ever been groomed by your gliders you know that it could do damage to an open wound.
Are your males all neutered? If they are then you may need to seperate your colony into pairs. If you are having dominance issues when they are neutered then they probably won't ever go away. And even if you are not hearing them fight it doesn't mean there isn't a problem.
First you need to seperate your injured male to a cage alone. Then you need to get him to a vet
and on antibiotics and that wound cleaned up. Then it'll probably take a month or so for it to heal up that he needs to stay seperated. Then you'll have to decide either put someone in with him or put him back in with the family.
They will not forget him, you can leave the cages beside each other so they remember each other if you are concerned.
I hope this helps.