As you've noted, he isn't listening to you. You need to DEMAND his attention. Keep him in the rope halter, on a good rope lead. Wear gloves. If he's behaving impeccably (not stepping into your space, moving forward but at a comfortable pace, looking forward and acting relaxed) leave the rope slack and have no pressure on the halter.
But as soon as he dips his head for grass, give a good yank up and say "UP!" until he stops doing it. If he balks and doesn't walk forward, another quick tug and say "Walk on!" until he does, reinforcing with a flick of the end of the rope near his hindquarters if necessary. If he tries to get way ahead of you or take off, give one BIG jerk and say "OI!" in a deep loud voice (don't scream it, bellow like a bull). Any time you see his attention drift elsewhere, give a bit of a tug on the halter and say "oi!". If he turns his head toward you in mouthy sort of way or is stepping into your space, start flicking a loop of the lead near his muzzle. Not going AT him, just drawing a line to indicate "no going past this point, buddy". So if he does step in, he gets a little whack on the nose (not hard, but disconcerting) but it's entirely his own fault.
Keep that up, then start trying just the voice cue. Pretty soon he shouldn't even attempt anything at all. And if something really does scare him, a quick "oi!" and little tug will soon get his attention back and stop the spook.
I don't mean to be a downer, but any horse with as serious a lack of manners as this one displays should NEVER be allowed to graze in hand! I wouldn't take this horse anywhere NEAR grass until he yields completely to you in an enclosure that doesn't have any grass at all. And even then, I wouldn't let the horse graze on the lead at ALL for a very long time. If the horse can't respect you when there are no distractions, then it can't be expected to respect you when there's delicious, sugary, wonderful green grass just below your boots.
Never once saw you ask the horse to move away from you in the paddock... this video means less than nothing because you never asked the horse to do more than follow the treat giver. Don't mean to be mean, but that's how it is.