1, Hey Zak, I'll try to keep this answer short, although its going to be hard to do!😂. Lets start with the easy stuff.
Have you had his teeth checked? He could have some sharp points or a wolf tooth bothering him. Does the bit have a pinch point or rough edge that could be causing some pain? If you can check both of those boxes, then you can go to training on him.
I would start from the ground. Make sure you can flex him easily from side to side in a snaffle. Is there resistance on one side or both? If yes, stay with it until he easily gives to bit pressure on both sides. When you have him soft laterally, you can start ground driving him.
Replied on Horse rears a bit when asked to back up
07 Dec 07:12
2, I like to put them in a round pen and drive them in a circle around me with the outside rein draped around their hocks. Send him around to the left at a walk. Make sure he is traveling freely and is not too concerned with all of it. When he has gone around the pen a couple of times, slowly draw the outside rein in until he has to turn to the right. If he stops, just hold light pressure and cluck until he completes the turn. Now go to the right for a while, maybe 1 full trip around, and draw him around to the left using the same pull and cluck. Now your going to the left. Go around about 10 steps and draw him around again to the right. Go 9 steps to the right and draw him down again and go 8 steps to the left. Are you tracking me? Just keep drawing him through those turns and making the distance shorter until he is facing away from you.