Work in hand get your horse to relax his back ask for forward down then some lateral bending at standstill at frost then walk. Do it on a rail to help him stay straight at first then do it on a circle make sure then inside hind is stepping under. Don’t use side reins or bungee as these are false aids for quick fixes if you do it right and take your time the rewards are great.
Trying bending circles. Practice makes perfect and I found with my horse the more I practiced the better he got. Just make sure to keep things interesting. Do different size circles for more flexibility and make sure to work each direction the same