Hi Maryann,
Please find below some amendments to the earlier posting.
Note that if you are extending the probation of an employee, it means that there is a problem you want to deal with. It is, therefore, fair that you give reasons for the extension so that the probationer can improve upon his or her performance.
This letter should not be in memo form but in the format like the appointment letter. "Yours sincerely" was used because he is currently a staff and not, in the strictest sense, an outsider. Also note that these are the new trends in letters with salutations like Dear Mr./Mrs./Dr./Rev., etc.
You should always put your name and designation.
Please let me know your reactions.
Paa Kow