[HLin NC]

No, he did not become a substitute by stepping off the field.

2012 Case Book 3.7.3 SITUATION A: B11 mistakenly believes he is his team's 12th player and leaves the field before the snap ... or (b) on his sideline
and enters his team box. B11 then discovers his error and returns to field on
his team's side of the neutral zone before the snap. RULING: (3-7-2) In (b),
there is no foul as long as B11 remains a player. (2-32-1) If done intentionally to gain an advantage, it is an illegal participation foul or it could be an unsportsmanlike foul.

Any player or the head coach can call time out. Being a captain has nothing to do with it. Being inside the nine yard marks has nothing to do with it.

As to the nine yard marks the rule indicates that if he was an A player in the previous down or an entering substitute, he must have momentarily been inside the marks at some point after the RFP and before the snap. Given a strict reading of the rule and no case play regarding it to the contrary, it appears he would not have to go back in the nines, if the RFP had been blown. However, if he camps out at the sideline or otherwise looks like he's got something to hide, one would have to consider that he was intentionally attempting to gain advantage as a pretended substitution and thus would be guilty of ill. participation.