Originally Posted by horatio
He sees a young Kirk, assumes that he has travelled back in time (without any alternate reality complications, just "there and back again"), believes that Kirk has come to rescue him ... and naturally assumes that Kirk is the captain if he comes for him.
He is a Vulcan but that's a thought of the moment thing and he surely can't tell by looking at Kirk whether he is 25 or 30.
Spock would have noticed the toupee! Hee Hee!