I know that plenty of people disagree but I think that issues that go unaddressed on the ground will manifest under saddle. He doesn’t respect you and has gotten away with ripping the line out of your hand a number of times. Ultimately I think that comes down to not respecting your forward aid and it will reappear in a nastier form down the road. I hope not but personally I would want to fix the attitude problem now and not later.
I really agree with the person who said that it comes down to the angle of lunging.
The only way a horse can get away is by turning the shoulder away from you and running. The horse can either immediately do that, back and then spin, or yield out and spin, but ultimately the shoulder and head turn away from you and then it is all over
If you control his shoulder you control his body. A horse can rapidly yield sideways but there is no way that he can yield his whole body sideways fast enough and long enough to get away or back away fast enough. He may be able to do both quickly and for an extended period of time but if you just keep trotting after him he isn’t going to back himself for a mile.
I was taught to lunge with the whip behind the girth but for most people the whip inevitably is pointed more at the hip than an inch behind the girth. I understand that this works really well for people but if a horse decides to swing his butt out, the whip pointed at the hip then becomes an aid that drives his hind end away from you and you lose your angle of control.
Random youtube clip I pulled but is this what you are experiencing?https://www.youtube.com/watch?v=yaVrRjjzbXc
You can see how the horse thankfully didn’t want to truly leave town because she had no leverage trying to pull a horse forward who was rapidly backing up. If she had instead hoofed it and gotten to a point where she was perpendicular to the shoulder she could keep pushing him out until she had yielding him to the point where she was back in control.
I am certainly not sitting here pretending that if I took over the lunge line I could presto magic fix your problem. However, I do think that controlling the shoulder is one of the most viable ways to address the issue. Increasing the leverage on his face is liable to make him fly backwards and then spin off or fly sideways and then spin off.