Issue of appointment letter and conditions for the continuity of the employment are two separate things and need not be clubbed as such.
It is mandatory to issue the appointment letter whether you keep any employee under probation or not. Therefore, you can go ahead and issue appointment letter with the conditions about continuity of services. Nevertheless, communicate to the employee on how will you measure his performance. This can be done either in the appointment letter itself or through separate letter.
6th December 2018 From India, Bangalore
7th December 2018 From India, Mumbai
I suggest following:
1. Have discussion and understanding with the candidate that his appointment will be initially on temporary basis for three months.
2 Convey to him that if his performance is found up to be of expected standards, his service will be continued with fresh appt letter on probation. Or else it will be terminated.
3 Explain clearly his job responsibilities and what end results expected from his performance.
8th December 2018 From India, Mumbai