Age-discrimination worry: Is it legal to ask 63-year-old about his retirement plans?

Q. We have a long-time employee who will accumulate the necessary points under our retirement program to become fully vested in his retirement benefit on his next birthday, which is in April. At that time, the employee will be 63 years old. He has not talked about how long he intends to continue working or his plan for retirement with our management team, which is concerned about having enough time to transition the employee’s work in the event that he abruptly retires. Can we ask this employee about his retirement plans without creating a claim of age discrimination? (Of course, the employee is also having performance issues, and management would prefer that he retire upon vesting in the retirement program.)