I see that you and Sky got some thoughts going. I pretty much agree with Sky.
As for finding a positive place to end the excersize, it doesn't necessarily have to be that he does your excersizse successfully. But rather that he does SOMETHING right.
I am not a trainer, mind you. And I've never, ever started a young horse. So, please take these musings for their limited value.
I think that the quality of a horse's response to what we ask is more important than that they do the excersize. So, if when teaching a young horse, who may have both a lot of confusion and some amount of resistance, if you get to the point where he is leading, maybe, but he is becoming resistant or upset out of confusion, then I think finding something that he can do and do calmly and willingly (the qualities you want to have in him), then that would be the place to go to at that end of your training session.
I had an old trainer tell me that how the horse was feeling at the end of the session was more or less the way they'd feel about you when you started with themm the next session.
It's really hard for me to say, without seeing the colt, whether or not he needed some more pressure coming up behind him (like Sky says), in order to break him out of being stuck , or not. It's a perfectly valid part of training to use some kind of pressure to get the hrose to break out of stuckness and move his feet. You always want the horse to be thinking that FORWARD = Freedom. The hrose that learns to stand still , back up or otherwise shut down the forward, is the horse that can have rearing problems , baulking, and maybe bucking. They forget to move forward when in doubt.
However, if the horse is frightened and confused, then maybe backing down to getting smaller changes from you asks. Like , you only ask the hrose to bend to one side and take one step off to the side. A horse that has stuck feet and is resistant to being led straight forward will often come out of that by being asked to move off to the side. They kind of have to to keep their balance. So, when they take that step off to the side, they get a lot of release on the rope and some rest time (20 seconds or so ) where you just let them stand and do nothing (their reward).
It depends on the horse.