Also, you can start with just leading with his harness on. You can practice the walk (lead), halt and stand when you lead him properly from his left side. Then switch sides and practice the same thing leading him from the right side. If your horse can't lead and halt quietly while on a lead line (draped loose), he'll never stop and stand quietly while ground driving.
Next step - while you are leading him ( you can handle the drive lines or not, your choice), ask him to step up and walk on while you drop back by his shoulder/withers. Then ask for your halts that way - with him standing slight ahead of you. Practice it from the right as well.
with our most up-tight mare, we spent hours (months!) - leading her asking her to stand with her head in the corner, stand w/ her head in front of the fence or to stand facing the barn wall or horse trailer. We also spent a lot of time leading and ground driving her - even after she was going in harness - with a "block" in front of her. When we'd stop/halt, we'd practice touching her (before blinders went on and also after we used blinders) and tugging on the harness. And when we got to the point where we were ground driving her, we'd often halt and then move to one side or the other and fiddle with the harness. They do learn. It takes time. When we started hooking her and we went to our driving lessons, sometimes she'd not want to make a turn and the instructor would step in and head her and lead her up where she needed to be/go and we'd continue. We went thru this a lot coming around part of his barn/trailer - of course - when continuing down the drive would have put us out by our trailer in the field away from the barn. We also did the technique of working pretty hard around the triailer or where she wanted to go and then going where I wanted to go to stand quietly and "blow" (breath for a while). I chose a specific "break point" and that was where we stopped for rest each time. That spot got further and further away from where she wanted to be.
If you haven't done so already - introduce obstacles that he has to step on, thru or over. After working thru rushing thru or over or jumping it until he walks only, ask for that all important stop. Count 1. Walk off, thru, over etc. Continue until he will stand for as long as you ask...
I was the one not prepared for this one - it caught me by surprise when my instructor got her to step into it and stand there. O - and notice that he is doing a combination of ground driving from the bridle, bit and lines and leading.
Obstacle work - including stepping into this tub or into a box or into a toddler swimming pool or onto a tarp, bridge etc teaches many things. Teaches them to slow down and relax, patience, to pay attention to what's under foot. Teaches you to slow down, take a breath and relax, work to teach your horse something new and so on.
I had started combo ground driving and lounging her in January. She was already trained to ride - both while led and off line in the round pen (her rider didn't graduate from the round pen before they moved to another state). She didn't like to stand still that way either - often spent time tied to a post or the trailer while I worked with his brother on a different pony or with his two step sisters on theirs. The above pics were taken in June.
This is her 3rd hitch to the cart - where he stepped in to guide her around the turn for me. This is August 10th. It would have worked better if I hadn't let the lines go so slack, but I did release them because he took hold of her bit/line not the halter under her chin.