Can we demand a second opinion on fitness for duty after FMLA leave?

Q. We have an employee returning from a leave taken under the FMLA. His physician has issued a fitness-for-duty certificate. However, we question the worker’s ability to perform his old job because the length of his absence was too short for him to recover completely. Also, the fitness-for-duty certificate simply states that he is “able” to work, without addressing his specific job duties. Can we send him to another physician for a second fitness-for-duty examination?