Question: Why does the Sefer Hachinuch (mitzvah 2) hold that the mitzvah of mila is not upon the father once his son is bar mitzvah if it was not done yet but in mitzvah 392 he holds that pidyon haben is still a mitzvah on the father even if he didn’t do it after his son turned bar mitzvah?
A. The Magen Avraham (Orach Chaim 444:11) says that every second one does not have a bris is a transgression (non fulfillment) of the positive mitzvah.
B. The father has to do pidyon haben to the son even if the son died (after 30 days of life) and the mitzvah wasn’t done yet.
Answer: As long as a man is an aral, uncircumcised, every second with foreskin is another transgression of a new positive mitzvah so when he turns 13 it’s a new positive mitzvah so the bar mitzvah boy takes this new mitzvah and the father is done. But by pidyon haben the obligation is not due to the fact that the son is not redeemed yet, for even if the son dies the father must still do the processes of redeeming him to perform the mitzva, rather the Torah requires the father to redeem him, and if so the obligation isn’t one the renews itself constantly, rather it is a one time obligation that starts when he is born (after 30 days of life) and lasts forever as long as it is not done. For this reason the mitzvah isn’t being newly forsaken every second, rather one has to get it done as soon as possible, therefore even once the boy grows up the father is still obligated since the original mitzvah has not gone away yet, and even if the father dies, the son just takes over the obligation of the original mitzvah.