Arranged marriage?
Sperm bank?
Bad merging?
Possibly there is a father and son both named Alexander Cope?
Alex I born c.1390, marries in 1424.
Eldest son is born 1425 and named after father, so Alex II.
Alex I dies in 1451.
Alex II marries sometime in the 1450-1470 period.
Alex II has son named William in 1470.
No sources just wild imagination.
Clearly the current details are completely wrong as you have pointed out.