The photo is a pretty blurry, but it looks like a wound with inflammation extending from the open area. What were his tank mates? I can't say definitely that this is a wound, as blurry as the pic is, but it looks like the outer layer of skin has been torn off, maybe by a bite, and that you are seeing the underlying tissue.
See if you can get better pics. As was mentioned by tolawdjk, step up the water changes to at least 50% - more if possible.
There's a thread on AC that chronicled the recovery of a Paroon shark that had huge areas of skin torn off and tissue avulsion; I wouldn't have believed he could recover from the severity of his wounds, but daily huge water changes with carefully conditioned and temp matched water was all the guy did and his shark recovered completely.
I had a seriously injured Oto that I did the same for, even 60 -75% water changes and he recovered completely. I did add some Melafix to his tank with each water change, and though some folks don't think it helps, it does have a mildly antibacterial affect; I know because my beneficial bacteria took a hit. The bb recovered, however, in just a few days.
You have to watch for signs of bacterial infection, which, at this point, I'm not sure is there, though redness can be a sign of infection it can also just be imflammation in surrounding tissue due to the injury.
Try to get some sharp pics if you can and let us know the input from the LFS people; seeing a fish in person is always best when trying to make a diagnosis, but often the LFS people are not always the most knowledgeable about that. The folks here on AC usually are much more experienced, really, and have better knowledge of illness and disease, diagnosis and treatment.
Again, this may be someting else entirely, but without being able to see the area more clearly my best guess is injury from another fish or getting stuck in a tight place.
Keep us posted. I'm wishing the best for your guy.