And thou shalt shew thy son in that day, saying, This is done because of that which the Lord did unto me when I came forth out of Egypt. And it shall be for a sign unto thee upon thine yad, and for a memorial between thine eyes, that the Lord’s law may be in thy mouth: for with a strong hand hath the Lord brought thee out of Egypt.

Exodus 13:8-9.

The father is to show his son something the Lord did to him (the father) in accordance with the Passover when he came forth out of Egypt. Some Rabbis tie this to the feast of the unleavened bread. But that's not something done to the father. Neither can unleavened bread be a “sign” upon the yad יד (the Hebrew word yad יד is used for either the "hand" or the "phallus").

Exodus 12-42-48 appears to present undeniable proof of what's being spoken of a few verses later in Exodus 13:8-9:
. . . this [Passover] is that night of the Lord to be observed of all the children of Israel in [all] their generations. And the Lord said unto Moses and Aaron, This is the ordinance of the passover: There shall no stranger eat thereof: But every man’s servant that is bought for money, when thou hast circumcised him, then shall he eat thereof. . . All the congregation of Israel shall keep it. And when a stranger shall sojourn with thee, and will keep the passover to the Lord, let all his males be circumcised, and then let him come near and keep it; and he shall be as one that is born in the land: for no uncircumcised person shall eat thereof.

Exodus 12:42-48.

Nothing seems clearer than that the primary thing that must be "observed" regarding the Passover (observed by all the children of Israel in all generations) is circumcision. If one is not circumcised, they are a “stranger” to the covenant, and cannot observe the Passover. Circumcision is the initiation into the mystery of the Passover. It clearly appears to be taught as the sign of the Passover that will be observed in later generations. All males must be circumcised. And then they can eat the Passover meal.
And thou shalt shew thy son [his circumcision] in that day [a future Passover], saying, This is done because of that which the Lord did unto me when I came forth out of Egypt [I was circumcised so that I could eat the Passover meal]. And it shall be for a sign unto thee upon thine yad, and for a memorial [zikkaron] between thine eyes, [so] that the Lord’s law may be in thy mouth [since you have the mark qualifying you to eat the Passover meal]. . . ..

Exodus 13:8-9.