I think there’s a reason why in Europe, it’s more common (or seems to me, anyway) to back the babies in the fall of their 3-year old year, then turn them out for another six months or so to mature.
I rode a five-year old warmblood a lot like this one - not mentally mature, wanted to test everyone. I only rode him for a couple of months before they moved him to an eventing barn (where they fried him asking too much, too soon). It took a LOT to get him to listen, and he was already fairly solid under saddle.
I never started a young horse where I didn’t have a very trusted and experienced person on the ground. That person was in charge for the first several times under saddle as the young horse got used to my weight and movement. They were also solid on lunging and voice commands. I would use the aids in conjunction with the voice commands. Gradually, the lunge line would be taken away, but not until the youngster was solid.
With yours, I would absolutely second the idea of ground driving him for another couple of months. Then turn him out for the winter, and let him grow up a bit. In the spring, continue lunging/ground driving him for a bit more, then start light under saddle work.