I think that there are a couple of issues at play here. One particular item is that Orwell makes it clear that whatever designations that Napoleon and Squealer confer upon the animals is secondary to what is actually believed. Boxer is not seen as a hero because he receives the award. Orwell does not capitulate to whatever leadership says. His creation of such a vision of leadership is made with this in mind.
For Orwell, Boxer is heroic in his self- sacrifice for the animals. Boxer's blind faith to his leadership is what enables him to give his life for the farm. Boxer's belief in the revolution and in the other animals is what causes him to do great things. Certainly, this is heroic. Yet, Orwell does not automatically persuade the reader that Boxer is heroic. This blind faith, the reason for his heroism, is something that is taken advantage of by those in the position of power. Boxer never questions, never wonders why things are the way they are, and never voices dissent. Certainly, Orwell holds the ability to question authority and to voice dissent as more worthy of praise than blind obedience. Hence, while Boxer does many great things that are heroic, he is not a hero in Orwell's eyes because of his blind loyalty towards those in the position of power. Orwell is more likely to confer the title of "Hero, First Class" to those who voice dissent and do something to actively raise awareness to the unfair condition that those in the position of power perpetrate upon an unwitting populace.