In Animal Farm, why doesn't Boxer escape when he finds out he's being taken to the slaughterhouse?

Prior to being sent to the slaughter, Boxer was loyal and perhaps the most devoted member of the farm to the Animalism movement. After the rebellion, he adopts the motto, "I must work harder" and eventually, "Napoleon is always right." He literally sacrifices himself to see the windmill project come to fruition. While working on the windmill tirelessly, Boxer collapses. 

Late one evening in the summer, a sudden rumor ran round the farm that something had...

Prior to being sent to the slaughter, Boxer was loyal and perhaps the most devoted member of the farm to the Animalism movement. After the rebellion, he adopts the motto, "I must work harder" and eventually, "Napoleon is always right." He literally sacrifices himself to see the windmill project come to fruition. While working on the windmill tirelessly, Boxer collapses. 

Late one evening in the summer, a sudden rumor ran round the farm that something had happened  to Boxer. He had gone out alone to drag a load of stone down to the windmill. And sure enough, the rumor was true. A few minutes later two pigeons came racing in with the news: ‘Boxer has fallen! He is lying on his side and can’t get up!' (46)

Boxer is hurt but committed to the cause. He tells the others, "It doesn't matter. I think you will be able to finish the windmill without me." Although he is devoted to Napoleon, he is no longer useful to him. In fact Boxer will cost Napoleon more now because Boxer cannot contribute to the farm. Boxer does not know this because of Napoleon's false promises of a retirement for older animals. Boxer is blinded by his loyalty.

Boxer is a loyal animal and trusts Napoleon implicitly. He also cannot read, so when he willingly gets into the truck for the glue factory, he doesn't know what is going on. Benjamin the donkey is the first to realize, crying, 

‘Fools! Fools!’ shouted Benjamin, prancing round them and stamping the earth with his small hoofs. ‘Fools! Do you not see what is written on the side of that van?’

Although the animals warn him, it is too late. Boxer is already loaded into and locked in the truck for the glue factory. His strength is compromised by his failing health. His desperation is apparent by the sound of "tremendous drumming of hoofs inside the van, but he is too weak to break free" (47).

Boxer fought arduously to escape death. His death was a result of his devotion and loyalty to Animalism and Napoleon's cold-hearted and duplicitous nature.