OP, I only do trail riding and my horse is like yours in that he doesn’t like to stand still for very long. I have a couple of suggestions that might help.
First, add a step back to your whoas. Make it one smooth movement, with no pause between the whoa and one step back. Whoa-back-rest. And with the rest, a big release with the reins. Then, just before he decides to walk off, ask him to move forward. Do this every time you stop, and gradually lengthen the time you before you ask him to walk off. Don’t expect too much at first. Like others have said, this is new to him and he’s a little anxious about it. If he’s really antsy, you may have to ask him to walk off almost immediately at first. Soon, he should start anticipating the step back, and this gets him thinking back instead of forward.
Second, when he gets jiggy, try trotting him in figure eights. This lets him move his feet, which he needs to do, and it’s vigorous enough to burn some energy. And with figure eights instead of circles, he also has to think about changing directions. Do this until you feel him relax, and expect that it may take a long time at first–thirty minutes or more.
In addition to making your horse work when he does the wrong thing, look for ways to reward him when he’s good. Maybe let him lower his head and graze a little or grab a few twigs off a tasty shrub. Just be sure that this is your idea and not his. You don’t want to create a bad habit; you’re just trying to teach him that he can relax and enjoy being away from the barn.
Good for you, for trying to find a way to keep your older horse active. You said you solved these issues in the arena, so I’m betting you can solve them on the trail too.