My thought is that this is about the timing of the reward for standing, and the reward itself. A treat is way too big and disruptive, and by the time it is delivered I doubt he knows why he got it. 
I’m also on a shifty horse and standing still is a #1 lesson, in all situations and settings. Glad to see that you have taken the same approach. But I would ask, how long is the horse asked to stand and not shuffle even one foot when you are asking on the ground?
There is a different way of thinking about the halt without lots of verbals, petting and treats. A halt is not a halt unless the horse is holding himself in halt, without the rider applying pressure through longe line, reins, voice or petting. You can use all of those cues to keep his attention on the fact that he is halted, but really that next shuffle is just being contained by you, it’s still there in his mind. Or, he can learn to halt and turn off his inner shuffle/fidget. Instead of not-moving-but-almost-moving, he is truly altering his own mindset to hold himself still, waiting patiently for the next command. However long the wait is…
Work on a longe halt, and other ground halts, for a minimum 30 seconds, solid performance, and count the seconds to be sure of the length. When this is perfected, you are still, giving slack, and not giving reminders. My guess is that although you and he both feel as if he’s been parked forever in your current halts that work, you are actually only holding him in place for 5 or 10 seconds. That feels like a long time to active creatures, but he isn’t really in a halt mindset. Work up gradually, from 3 seconds, to 8 seconds, to 15 and so on. That will introduce the mindset of a true self-contained halt (to both of you).
Make sure that whenever a halt is ended and he moves again, it is you who end it and move him. Never him. Read his body language, and even if he’s only been halted for a half second, if he’s about to move and you can’t stop him, then you move him first. Makes sense? He never, ever is the one who initiates a move. Only you. With major manipulation! 
Also, halt exercises are not true unless he is standing with no pressure from you. No tension in the line or the rein. No verbals or petting. Those things are not really rewarding him, insteady they are holding him in halt, so that he isn’t holding himself in halt. Without that pressure reminder, then yes it is so much harder to initially train compliance. But that is the thing to work through, gradually. He isn’t truly, solidly parked unless he parks himself, as it were, without you anchoring him through pressure and reminders.
Reward with release, while he keeps himself in halt with the release. The release must be immediate to be recognized as reward. Recognize even the slightest instant of halt, every single tiny try or accidental stop. It takes absolute consistency and many repetitions to get it across, but be consistent consistent consistent. This is a major reward that he will want to repeat! I promise that he will respond better to the release reward than to all the “good boy’s” and pets. Don’t reward with a lot of verbals and petting, which are distractions, and ‘cheats’ as reminders that he is still being asked to stay, rather than true rewards that he recognizes (plus illegal in dressage competition). It’s like the pressure on the line, he isn’t really self-halted, you are holding there with these distractions. Instead, give some slack and let him stand on his on.
This will be a bit more complex than what you are doing now and it is in many ways a different way of thinking about what a halt is. It works, though !!! A horse that is self-halted and has it in his mind that he is still and waiting for you is an honest halt. (And then you train the prompt transition to the next gait, from the halt.
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It takes time, it takes perseverance, and it is so much more a solid gold extended halt than is sitting there at X on a horse that is about to do something other than halt! 
Before telling yourself that you don’t think your antsy horse can learn this (especially after the first couple of tries), go watch some reiner videos. Those horses are high-energy, they fly at the touch of an aid, whip around spins, blaze through the fast work. But when they halt, They. Are. Halted. They are on a totally slack rein, planted and going nowhere until the rider asks. Not because they are bred differently than your horse, but because they are trained differently than your horse. Anything they can learn, your horse can learn, too. 