Hi Timea, I agree with Tamara - trotting poles and raised trotting poles are great. My horse also loves loosening through his back with stretches on the ground using treats. Making sure he is engaging his hindquarters as much as possible on the flat really helps too, as working correctly over his topline will help to strengthen his back. I also due lots of long and low work, getting him stretching through his frame. Lateral work also helps my horse to round over his back. I hope these all help! My horse had issues with his back a few years ago, so I love using all of these!
Hi Timea, you could try trotting poles while lunging or riding. Start with just a few if your horse isn't uses to trotting poles, and work your way up to more poles (if they are available). Next you can raise the poles a little.