He shall be thy spokesman. Literally, "He shall speak for thee." He shall be, even he. It is the verb that is repeated, not the pronoun. Probably the meaning is, "he shall surely be." There is no comparison between Aaron and anyone else.
Thou shalt be to him instead of God. Divine inspiration, that is, shall rest on thee; and it shall be his duty to accept thy words as Divine words, and to do all that thou biddest him.