There are many ironies in Animal Farm and the removal of Boxer is a significant one. Old Major had warned Boxer early in the novel that his hard work would count for nothing. Ironically, it is the pigs who now dispose of him:
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
Benjamin, the cynic who chooses to watch as the pigs take over and is more like an observer than a participant, is unable to help his friend Boxer when he realizes what will happen to him. His call to the animals is too late and Boxer is unable to free himself. The animals allow Squealer to convince them, a few days later, that
Boxer died in the hospital after receiving the best care
and it was all a misunderstanding. Squealer explains away the fact that the cart displays a disturbing sign
Alfred Simmonds, Horse Slaughterer and Glue Boiler, Willingdon, Dealer in Hides and Bone-Meal. Kennels Supplied
by making up a story about the 'new' owner - the vet. The animals fears are allayed and the pigs take pains to show their appreciation of Boxer by claiming that they will lay a wreath on his grave. There is NO grave! A memorial service, for their fallen "comrade" will be held in Boxer's honor. Seemingly insignificant occurences containing much irony, revealing the pigs ultimate deceit and betrayal.
Napoleon wastes no time and, as an opportunist, uses it to reinforce the principles the animals should live by. All the animals respected Boxer so reminding them of Boxer's mantra to "work harder" and "Comrade Napoleon is always right" is the perfect platform to promote his form of 'animalism.'
The pigs will have been paid for Boxer's remains and the crate of whisky that arrives is doubtless purchased with the funds - the ultimate betrayal which adds to the irony surrounding Boxer's death and the significance of the memorial service.