I’m afraid we can’t speculate - particularly sight unseen - on what it might be or its cause. (Except if it is in the middle but low down on his back end, then are you sure it isn’t his grease gland?)
All we can advise is if you are concerned then to see a vet - a scab could be due to a fight (please see next paragraph), a burst cyst, parasites etc.
You can add a picture to this thread and we may be able to assist but we cannot diagnose anything.
If you have seen your piggies actually fighting and enough to cause injuries to each other (ie something more than normal dominance of mounting, chasing, rumbling etc), then sadly that means their bond is not a functioning one and they should, unfortunately, be permanently separated. I did see that you posted a few weeks ago about them potentially injuring each other so if you are finding more wounds then I would be concerned about their bond.