Here the 1874 RLDS edition replaced his with the, perhaps accidentally, although a possible motivation for such a change would be the following relative clause: the his is somewhat redundant when followed by “which he had spoken unto him”. Nonetheless, the text has several examples of this same kind of redundancy:
Nor did the 1874 RLDS edition change any of these other examples, which argues that the change in Ether 12:20 was accidental. The 1908 RLDS edition restored the correct his here in Ether 12:20.
Summary: Maintain in Ether 12:20 the occurrence of his word even though it is followed by a somewhat redundant relative clause.