He is decisive.
He is a persuasive speaker.
He is firm in his beliefs.
A person who is decisive is one who can make decisions and stick with them. Old Major has decided that "All men are enemies. All animals are comrades." He sticks with this decision which also shows that he is firm in his beliefs. This entire speech of his is meant to persuade the animals to believe in the same way he does. He uses a variety of techniques in his persuasive speech. And they work! At the end of the speech the animals give "a tremendous uproar." From this we can gather that is a persuasive speaker.
A person who is naive is unaware of the consequences of his/her actions or those of others. Cowardly is the opposite of brave.