What happens to Boxer and how do the other animals learn of his fate? How do they come to a final conclusion about these events?

is the
most tragic character in the book. He is hardest working animal and he does genuinely believe in
the revolution. His mottos are famously, "I will work harder," and " is always
right." 

As to what happens to him, there are two storylines. 

First, according to Napoleon, he is taken to this animal hospital and taken care of
before he dies. He was cared for, loved, and honored. Napoleon is obviously lying to make
himself look better. 

Second, there is what really happened, which is
intimated early on whenpredicts what Jones will do:

You,
Boxer, the very day that those great muscles of yours lose their power, Jones will sell you to
the knacker, who will cut your throat and boil you down for the foxhounds.

After Boxer serves the farm with all his might and grows ill and
cannot continue to work, Napoleon sells Boxer to the...