"his ear": His right ear is intended. You say, the right ear, but perhaps it is the left! — It follows (that the right ear is meant.) It is written here "his ear," and elsewhere (Leviticus 14:25, in respect to a poor leper,) "his ear." Just as there, the right ear is specified, so, here, the right (is intended).
When the Torah Describes the Ear-boring of a Hebrew Bondsman
Mekhilta Tractate Nezikin 2:19