Those pics are pretty hard to look at. I feel so sorry for him and for you, buddy.
To me, I don't really see conclusive signs of infection, yet.
It looks to me that, in the pic below, the areas I have circled in red are where the fat layer that lies under the skin and over the deeper muscle tissue has been either bitten off, sloughed off, or erroded down to the muscle fascia or covering.
I think what you are seeing that you take to be some kind of fluffy bacterial or fungul growth, is the fish's adipose tissue or fat.
The areas circled in orange looks like the roughened, globular looking surface of the fat layer where the skin has been torn off but the layer of fat is not totally destroyed.
Either area could, however, be the beginning of granulation tissue. Part of the healing process.
The reddened areas circled in red could be the beginnings of the process of granular tissue forming over the muscle fascia. A raw open wound that begins to heal does tend to get red, rough and beefy looking.
Granular tissue usually starts forming at the wound margins, growing towards the center, though it can form over the central area and progress towards the wound margins, too.
The process of healing starts with the inflammatory process. Blood vessels dialate which increases the O2 to the tissues, and brings white blood cells, serous exudates, and nutrients to the area, and all that helps granulation tissue formation. Of course that's kind of a simplified explanation. I know that so much more is involved in wound healing and I'm not a doctor or a veterinarian.
I don't know how the dynamics of being constantly bathed in water, and living in it as a fish does, complicate the process, but I'm sure fish have been able to heal with very serious wounds. In fact, the immune systems of some animal species seem to far exceed the capabilities of the immune system of humans.
That's why allowing the fish's immune system to do it's work, in as pristine as possible (of course not sterile), physically and mentally stress free environmen, has seemed to be the best hope for your fish as suggested by DrVader and KarlTH, and has been my thinking, as well.
My opinion is not worth a whole lot though, lol, I'm still pretty new to aquaria, though I do have a smidgen of knowledge of medicine in humans.
I still feel stongly that the application of a topical antibiotic on such deep tissue injury is inadvisable. I think smaller wounds, yes, but in the case of this fish who has such emormous areas of injured, exposed flesh, no.
Since you are in constant attendance with the fish you could watch carefully, wait to see the earliest signs of infection, post pics for consultation and if needed start treatment with broad spectrum antibiotics.
Or on the flip side, go ahead and try to prevent it in the first place, as other equally experienced fishkeepers have advised.
The flaps of skin are worrisome. If he were a human the doctor would want to debride flaps of skin that are at the wound margins that are not going to cover the denuded area and are, therefore, not going to be a part of the healing process.
I worry that the hanging skin may impede the healing process: the movement and flapping of the skin possibly tearing the skin away from the area even more.
I read a fascinating thread in which a goldfish had broken his pectoral fin. The area of attachment at the fishes body was in danger of tearing, in fact I believe, if I'm not mistaken, it had started to tear a little.
Flarringshutter posted in that thread suggesting that the torn fin be amputated to avoid further injury to the fish.
In the post she recommended anesthetising the fish with clove oil to amputate the dangling broken fin with sharp cuticle scissors to avoid more damage to the fish. (ouch, the clove oil would be painful in this situation, but maybe another anesthetic can be obtained)
She gave step by step instructions on how to do this.
I'm not advising this drastic step, but I wonder; should the loose and flapping skin be cut off; a surgical debridement of the skin flaps and tags that are dangling? Would this improve the healing process?
Certainly the other side of the argument is again, that this would be very traumatic for the fish, even if eventually anesthetised, as the process of catching him and putting him into a tub of some kind to anesthetise him and carry out the procedure would be very stressful.
Having said all that, at this point it is a coin toss as to wether you will have a better chance of success by allowing the fish's own immune system do its work, or whether you should attempt to help it with some kind of antibiotic in the water.
Will the antibiotics put increased stress on the kidneys and liver of a seriously ill patient and do more harm than good?
Will waiting allow a fulminating infection take hold.
It is true, that he would be on IV antibiotics in the ICU if he were human, or even any other land dwelling animal at the veterinarian hospital, but kidney and liver function would be monitored as that is carried out.
In the end, should your fish die, in spite of your hard work and best efforts, and all our best wishes and sincere efforts to advise soundly, we will always wonder if the other treatment options that you did not choose would have worked better.
Hindsight.....you know....
Sorry to ramble on so... I'm no help, really