I think one of the most important ways that Orwell establishes massive reader sympathy for Boxer in his novel is by consistently portraying him as the hardest working member of the farm and highlighting his constant self-sacrifice that he makes on behalf of the farm and its various projects. For example, when Boxer collapses in Chapter 9, note how his first thoughts are of completing the windmill:
"It does not matter. I think you will be able to finish the windmill without me. There is a pretty good store of stone accumulated."
He out of all of the animals has been the most loyal (and therefore the most manipulated) to the farm project.
What also adds to the sympathy we feel for him is the addition of Boxer's plans for his retirement:
Boxer professed not to be sorry for what had happened. If he made a good recovery he might expect to live another three years, and he looked forward to the peaceful days that he would spend in the corner of the big pasture. It would be the first time that he had had leisure to study and improve his mind. He intended, he said, to devote the rest of his life to learning the remaining twenty-two letters of the alphabet.
By revealing Boxer's plans in such a way Orwell ensures our anger that he is robbed of the peaceful life he deserves after all of his toil, especially when we find out that Napoleon has sold him to be slaughtered.
Thus Orwell creates tremendous sympathy for us by describing Boxer's unstinting work and the way that he is exploited by Napoleon and denied his long-awaited retirement only to be callously disposed of.