What happens to Boxer and how do the other animals learn of his fate in Animal Farm?

Boxer had suffered a split hoof, but he seemed to be
recovering and contined to work harder than any of the other animals on the windmill.
But they noticed his hide was no longer as shiny as it once had been as he approached
his 12th birthday. One day, it was reported that he had fallen and could not get up. It
was his lung, Boxer told the animals. He was looking forward to his retirement, and it
was assumed that he would live out his last years in restful solitude. Squealer promised
that the local veterinarian would heal Boxer, but one day the animals saw an enclosed
wagon taking Boxer away. The animals bid him goodbye, thinking he would soon be healed.
But Benjamin read what was on the side of the
wagon:

"Alfred Simmonds, Horse Slaughterer and Glue Boiler,
Willingdon. Dealer in Hides and Bone-Meal. Kennels
Supplied."

It was too late for the animals to stop the
wagon, and they watched as Boxer was sold to the glue factory.