Answer

By Your Horse

Instructor Alison Ritchie says…

He needs to go back to basics because he’s probably forgotten most things, and you need to show him a new way of being. In-hand work would be good – teach him to move forwards and backwards (this is particularly important in confirming the half-halt) and sideways, from your body position. Carry a whip to lightly tap him where your leg would go and reward all good behaviour.
Lungeing and long-reining would also be good for him, and some work in side reins will encourage him to work in a better posture. To stand up straight he needs discipline and muscle tone. He’s been off work for a while and so it’ll take him time to develop the discipline and muscle to hold himself there. This is why he runs through your hands. We have to be realistic about what our horses can achieve. Your boy is probably more of a rugby player than a ballet dancer! That’s fine, just don’t expect the sort of collection, especially not yet, that a finer horse with different conformation may achieve.
I’m sure with the right training you’ll be able to get a better response while still training in a snaffle, which means dressage tests are still within your reach.