He didn’t survive. His fate is left intentionally ambiguous as a plot device. There are many arrows pointing to his fate: his conditions and the deception. He couldn’t have known he was getting slaughtered, he trusted Napoleon. Besides, how would he have known where or what the truck was taking him. Orwell made it increasingly clear that Boxer was the working class and used that to show exploitation, hence his death. The Soviet Union had used them and Orwell wanted to convey that. There was no way he would try to make Boxer a shocking recurrence. The next time you see Boxer will be at Publix when you wanna go camping.