During probation review, you must show that a staff is incapable to deliver the required task (with real facts/data/record), then put him in PIP. During periodic review, you must show that he still can't achieve the targeted work performance. then the final review can terminate him. By doing these steps, at least your base is strong enough to defend if he brings up this case in future. most important is to have every review, comments, recommendation written in black and white, and all parties involved to sign on it.