The main verb of your sentence is the third person singular present tense of prove, which belongs to a class of verbs in English that are followed by a direct object (often a reflexive pronoun like himself in your example) along with an object complement, i.e., a noun phrase that corefers to the direct object; that is, in your sentence an equivalence is being made between himself and a master of rhetoric. This is probably why the to-infinitive of the copular verb be (i.e., to be) often occurs between the direct object and the object complement in that the semantics of prove X (to be) Y entails that X is Y. In other words, Douglass, the referent of himself, is (third person singular present tense of be) a master of rhetoric, as asserted by some fact or agent that "proves" this.
Other verbs that can be used in this same way as prove are consider, show, believe and think. And I agree that to be can be omitted from Frederick Douglass proves himself to be a master of rhetoric with no perceptible change of meaning but a desirable gain in the conciseness and economy of the sentence (which I could use a little of there, i.e., I just mean the sentence has fewer unnecessary words) while remaining grammatically correct.
That's the grammatical viewpoint, now rhetorically there are some separate issues:
- I agree with Mark M. that maybe prove is too strong a verb here, though it's a bit perilous to judge outside the context of your sentence. Presumably, since you included the tag "English Literature" in posting your question, you're discussing one of Douglass's many writings (I assume also one of his non-fiction essays or autobiographies--I confess I am ignorant of any imaginative literature he may have written) and you're trying to argue that his rhetorical style in the work illustrates his "mastery." If so, then I think along with Mark that it might be a stretch to declare that ontologically and with certainty a writer's rhetoric "proves" anything. Instead, perhaps a less definitive verb like demonstrates, shows or illustrates would be better.
- And if you are discussing a work by Douglass that is freely available and readable by others in the present time, I would say that present tense proves is appropriate--if not even mandatory--even though the author himself is deceased.