Put words in his mouth, i.e. instruct him what to speak, and command him freely and faithfully to express it. See Isaiah 51:16, Isaiah 59:21.
Related Commentaries of Exodus 4:15
Exodus 4:15
15 And thou shalt speak unto him, and put words in his mouth: and I will be with thy mouth, and with his mouth, and will teach you what ye shall do.