If you have trained difficult horses, this young horse with an absolutely clean slate should be easy!!
One thing I would not have done was to have immediately roped and forced a halter on him. I would have left him in the round pen, fed, watered him, and let him be to settle down. Why rush that, when you have all the time in the world? Let him settle in for a few days, let him realize you're not going to kill him, and he probably would have come around pretty quickly...NOW, however...you have pushed him to his fight or flight instinct, and you have to figure out how to overcome that.
You may have to use some round penning techniques to start him all over again. Don't push him as hard as you would an adult, since his joints can suffer more damage at his young age, but in order to get him responding to your body language and yielding to you (off line), that is where you will probably have to start, in order to start building some respect for on lead. Get him responding to your body without the restriction of the lead, first, then reintroduce it later, when he is no longer afraid or clausterphobic while around you. Then when you DO reintroduce it don't simply put pressure on it, to get him to follow you around, just work with him as if it weren't there; I start by getting them to yield their hips either direction, then their shoulders...then I will move foward a step, and then back to yielding the hip or shoulder, eventually they are following you around without even realizing really what has happened, and are very soft to the halter, because it's not really about the halter in the first place. Yes, they do learn that it is there and they are 'bound to you', but it doesn't come out of fear.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."