Did a bit of research in the OED, and here's what I found.

"[Reduced form of sire SIRE n., the shortening being due to the absence of stress before the following name or appellation. The forms sore, sur(e may represent OF. sor or sieur, the oblique case of sire.]"

Additionally, they attest "ser" and "sere" from the 14th century ("Sere biscop, ta god kepe," Cursor Mundi, c.1300; "The fayre tour..whech {th}ou say, Ser Pope, is {th}e grete excellens of {th}i dignite," Scott. Jrnl. Topogr, 1509). But they also attest "sur," "sure," "sore," "scher," "schyr," "schir," and "shir."

In conclusion, yes, you would probably come across some instances of "ser" in a late medieval text. However, you'd also probably come across every variant you could think of that went along the lines of S-vowel/vowel combination-R. Let's just say spelling bees weren't in vogue at the time.

EDIT: Also, while we're on the topic, Wikipedia is not your best place for etymology. Try the online etymology dictionary (www.etymonline.com) or, if you can get your hands on it (if you're in school you probably have access to it through your school), the Oxford English Dictionary.