I am failed to understand the view point of your Employer. An employee, after appointment is placed on probation for a period of 3 months or 6 months, or for one year (in some companies).
When we say a probationer, it means he is not fully equipped with (or rather competent enough) for delivering the goods/bringing results. It is during the probation period, an employee is learning about his job, his department, interaction with HOD and other department colleagues, work environment in the company, policies and programs of the company, HR policies, etc.etc
Periodical reviews /evaluation of performance are conducted by the management on bi-monthly /tri-monthly basis. Shortfall in performance are communicated to him and mentoring him to improve his performance.
So, when the performance of a probationer are not upto the mark, you can extend his probation.
Thanks.