Lunge work in a roller helps loads 👍🏼
Check out art2ride with Will Faerber
Long and low.
Make your reins long and then pull your hands out as far as you can and wiggle the bit to make the horse bring his head as far forward and as far down as he can(the goal is to get under his knees) make sure you use a lot of leg.
Do this at a walk and then a trot.
After he gets a hang of it shorten your reins acouple inches until you get to a very short rein. Basically where your collected and have him on the bit.
After this you can have him do long and low over cavaleties of 6 poles or more.