Ugh that's awful, he needs to get to the vet. From here it looks more fungal in nature, but with such a huge afflicted area like that we typically treat for all three since you'll tend to see them in constellation with each other once the initial problem gets that out of hand.
It's frustrating to see a pig with skin that bad since these issues often take weeks, if not months to progress to that point.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry