When I used to want my old horse to put his head down I used to loosen the reins then when he put his head down pat him to tell him he's doing the right thing. Gradually he understood what I wanted him to do and lowered his head without me haven to loosen the reins
Ugh, please don't wiggle the bit, or have your hands wide and low or any of that. Your horse will develop its headset over time, it shouldn't be forced into it.
What you should do is focus on bending your horse through corners. Think of your horse like a long piece of rubber. Your inside hand will apply pressure, use your inside leg to support the bend, and use your outside leg behind the girth to keep his haunches from swinging out. Do serpentines and circles in your arena to practice this bend.
Also, practice this on a trotting circle: try squeezing the inside rein like a sponge (use your leg to push him forward too), and when he drops, relax your rein forward. Don't get overly aggressive with this or your horse will start to dip behind the vertical which is just as bad as above it.
Practice this and your horse will naturally develop a proper headset, without being forced into an uncomfortable false headset. As you develop this, make sure you start slowly as he'll be unlikely to hold the position for very long. Let him stretch frequently
Wiggle the bit in his mouth
we actually do a lot of groundwork (at least 30mins to an hour during one lesson) but he just seems to throw his head up. I could be asking for the transition incorrectly, but he is never chill.
well he has broken his neck twice so I don't really know if it has something to do with that but ya😂💚
Ground work. Walk backwards (facing your horse) in a little bit of circle, and ask for a little bit of flexion on the bridle (or halter) and use a lead to apply pressure where your leg normally goes. When they soften, put them back to walking straight.
I have been working on this with my horse in a halter, and she improving.
Make your hands as triangle and use lots of legs in the walk then build up from there for the trot and canter work
To get my horse to lower her head i raise my reins slightly. She has an issue with putting her head up way too far so whenever she does that i just follow her. She learns that she can't put her head up to escape pressure and it rounds her out nicely.
Riding with your hands low and wider apart also helps encourage a lower headset :)
Sponge your reins with the horses motion. If that doesn't work talk to your coach. There's a horse at my barn that had that same problem and it's because he had been riden wrong for his whole life. Good luck!