Boxer's fate is that he dies of wounds that he sustains at the Battle of the Windmill. He might have survived, but Napoleon doesn't really care enough to get him medical help.
Boxer's character traits affect his fate because his main character trait is his unquestioning belief in Napoleon. Boxer is supposed to represent the working classes who blindly followed communism.
Boxer keeps working really hard even after he's wounded and even after his lung collapses. He truly believes that whatever Napoleon says is best so he just keeps on going.
This trust in Napoleon dooms him.