Yes he can
I believe he is within his rights as a Commander to implement this policy. It is a common practice to require a DA Form 4856 be used in addition to the mandatory counseling form. The purpose for this is that the DA FORM 2166-9-1 does not provide adequate space to address issues and concerns. It is usually more of a reflection of a working copy of the evaluation report rather than an adequate counseling addressing: areas to sustain, areas to improve, upcoming events, promotion potential, and education.