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 correlatives purpose is to express the characters
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 authors 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: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 childrens 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 OBrien 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,is showing us how Winston feels, rather than telling us.  Orwell is connecting
the ulcer to the journal and, later, to .  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