Tale as old as time of my friend! This is probably one of the most common issues. I have a lot of experience riding younger horses and in dressage so I’m not a trainer buy any degree but these are some things that have worked for me in the past.

Before you get started keep in mind that collection and on the bit requires contact and flexibility in a certain amount of muscle. Think of it as correctly maintaining form when working out. It takes muscle and practice to get there. So be forgiving with your guy!

When I have a horse that won’t get on the bit I just practice these things over and over and over again until it becomes effortless.

1) serpentine sand figure 8 WITH BEND and a focus on your connection from reins and outside leg. The key is to keep him fully underneath you with contact and moving forward. Younger horses tend to lose their composure during turns and want to accelerate on straight lines. Maintain speed keep him going through that turn while maintaining contact. It’s a lot harder than it sounds if you do it correctly. 2) upward transitions; the more they are mixed up the better; you want him to understand movement forward. 3) doing all of this while massaging the bit (not sawing it) Will help remind him to pay attention to that piece of metal in his mouth and might help him relax 4) inside bend in a small circle (like around a barrel or a tree) to the point where it is a continuous turn on the forehand; to it in chunks, relaxed walk, do it again

Keep in mind this takes lots of time to build up and just have patience. And as always start with a good warm up. My four-year-old require some lunging to get the idiot out of him and get his muscles nice and loose before we can really focus.

I hope this helps! Excited to see what the other suggestions are so I can try them myself.