Or HaChaim on Deuteronomy 15:17/אור החיים על דברים ט״ו:י״ז

1 לך לעבד עולם, "for you as an eternal slave." Seeing that if the original master had died during the first six years of said servant's service he has to complete his term working for the master's son, the fact that he had his ear pierced לך, "for you," means that he will remain yours (the master's) forever but not your son's forever.