"Ser" as a knight's honorific is, as far as I'm aware, original to George R.R. Martin's A Song of Ice and Fire series. The use of sir as an honorific title in English traces back to the late thirteenth century, before which sire was the most probable form of address. This was less of an honorific title, and more of an address. A contemporary chronicle would mention a twelfth century knight, say, as just 'Odinel d'Umfraville' rather than 'Sire Odinel d'Umfraville'.