A horse not leading is either an ignorance problem or a disrespect problem. When I teach a young horse to lead (or to do anything in a halter) I try to start off in a roundpen, a set of small cowpens, a small paddock, anything where he can't get too far away. I start off without a lead rope and send him in a circle away from me. My theory is that when you send a horse away, the only thing he wants is to come back. Go both directions at a trot or lope, making him change by stepping in front of his face. A few minutes after he decides he wants to be friends again, let him come into you (make sure he is facing you) and put the lead on. Do basically the same thing as before, but now it is like lunging since you have the lead. Make sure as he is going in circles his nose is tipped in and he is not pulling on the rope. You can work on that by gently giving and taking with the rope. I use the other end of the rope or a lariat to send towards the hind end to keep them moving. When you want to change direction, switch your hands on the rope, turn your body to face his hindquarters and tug a little at his nose. The goal here is for the colt to swing his rear end out and turn to face you. When he does that, send him out again on the other direction. Basically what you are trying to achieve is for your horse to be soft to the halter, responsive to your body language, and to look to you for when to move what part of his body.
With a big ole horse like that, I wouldn't want to deal too close up with him! That's why I like sending them off away from me. Less accidents!
I don't know if I explained this well, but I hope it helps. It works with all the colts I start.
Don't squat with yer spurs on!