"let him go"? You can't even do that as DS female... no matter what you choose, Carth gets killed, either by you or Bastila. Before I had realized there was a cut ending, I always thought "gee, what a waste; it sounds like he has something important to say but Bastila cuts him off and you never hear it.... and oop, now he's dead. "
From what I can remember, you can choose to let him go, and he just flies away and says he'll find some way of helping you.
Of course, I might just be mixing things up.