We are told his ulcerous sore "had begun itching unbearably." For what could this ulcerous sore be an objective correlation?

This is a tough one.

Remember, T.S. Eliot's formalist theory says that

The objective correlative’s purpose is to express the character’s emotions by showing rather than describing feelings...

AND

this action of creating an emotion through external factors and evidence linked together and thus forming an objective correlative should produce an author’s detachment from the depicted character and unite the emotion of the literary work.

Let's track the ulcer:

pg. 1: a varicose ulcer above his right ankle

pg. 4: Moreover his varicose ulcer had begun itching unbearably. He dared not scratch it, because if he did so it always became inflamed. The seconds were ticking by. He was conscious of nothing except the blankness of the page in front of him, the itching of the skin above his ankle, the blaring of the music, and a slight booziness caused by the gin.

pg. 17: His veins had swelled with the effort of the cough, and the varicose ulcer had started itching.

pg. 41: Winston reached down and cautiously scratched his varicose ulcer. It had begun itching again. The thing you invariably came back to was the impossi- bility of knowing what life before the Revolution had really been like. He took out of the drawer a copy of a children’s history textbook which he had borrowed from Mrs Parsons, and began copying a passage into the diary:

pg. 48: and his varicose ulcer was throbbing

pg 87: He had grown fatter, his varicose ulcer had subsided, leaving only a brown stain on the skin above his ankle, his fits of coughing in the early morning had stopped. The process of life had ceased to be intolerable, he had no longer any impulse to make faces at the telescreen or shout curses at the top of his voice. Now that they had a secure hiding-place, almost a home, it did not even seem a hardship that they could only meet infrequently and for a couple of hours at a time. What mattered was that the room over the junk-shop should exist.

pg. 158: Here and there under the dirt there were the red scars of wounds, and near the ankle the varicose ulcer was an inflamed mass with flakes of skin peeling off it. But the truly frightening thing was the emaciation of his body. The barrel of the ribs was as narrow as that of a skeleton: the legs had shrunk so that the knees were thicker than the thighs. He saw now what O’Brien had meant about seeing the side view. The curvature of the spine was astonishing. The thin shoulders were hunched forward so as to make a cavity of the chest, the scraggy neck seemed to be bending double under the weight of the skull. At a guess he would have said that it was the body of a man of sixty, suffering from some malignant disease.

pg. 160: They had dressed his varicose ulcer with soothing ointment. They had pulled out the remnants of his teeth and given him a new set of dentures.

Notice, Orwell is showing us how Winston feels, rather than telling us.  Orwell is connecting the ulcer to the journal and, later, to O'Brien.  The ulcer stands for emotional/intellectual rebellion, rather than itself.

"He dare not scratch it" is to say "he dare not write it" (in his journal) or "think it" (thought crime).  As he grows fatter, more comfortably rebellious, it subsides.  Then, after he is caught, O'Brien and the Ministry treat it, soothe it with ointment.  They torture him; control his body.  He no longer thinks or writes.  The ulcer never was.  I has become an unulcer.

Remember, a satirist must always concern himself with, according to Kubrick, "precious bodily fluids."  So, which would you rather have?

Emaciated and rebellious = itchy ulcer

Fat and rebellious = nonitchy ulcer

Emaciated and not rebellious = no ulcer