Bottomline
It depends
Discussion
If you truly are a volunteer, then he would have no right to demand you perform PT on the weekend with him. However this does not mean you should simply disobey him either. In this case the most prudent course of action would be to discuss the issue with your leader and let your leader inform the other NCO that you will not be participating in his weekend PT.
If however you are not a pure volunteer but were directed to participate in remedial pt then he would have grounds to require your presence provided the weekend PT is approved by the commander as a remedial training program. Again the best course of action is to take the issue up with your leader.
Furthermore this leader’s weekend PT might be overreaching his authority. If this is something he is doing on his own without Commander approval and he is requiring Soldiers to be present in their off duty time, this could be viewed as punishment which would be illegal.
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