The answer goes back to Genesis 3:15 -
> And I will put enmity between you and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel.” - Genesis 3:15 (NIV)
In this case God is speaking to the serpent in the Garden of Eden; the "he" is speaking of Eve's descendants (any and all of them) and hence including, in the story, Ransom. Weston thus represents - well, is - the metaphorical serpent.