"then he shall serve you": — and not your heir. I might think, not even your son; it is, therefore, written (Shemoth 21:2) "Six years shall he serve" (implying that he does serve your son). And why do you see fit to include the son and to exclude the heir (i.e., the daughter)? I include the son for he stands in place of his father for yeidah (living with his father's handmaid), and for (acquiring his) field of holding (viz. Vayikra 25:25), and I exclude the daughter, who does not.
If he ran away and returned, whence do I derive that he completes his (six) years (of service)? From "then he shall serve you six years." I might think that if he took sick and recovered he makes up for his days of idleness; it is, therefore, written "and in the seventh year you shall send him free from you."