We have a stretch we use on our older Oldenburg and it works on all horses to stretch their back up. You stand behind the horse and put your thumbs on either side of the croup, just out from the spine, about where his rump begins, and draw your thumbs down towards his tail. Your hands should be on either side of the tail. It makes them stretch their backs up like a cat stretch. Just make sure your horse doesn’t kick. Usually the first time it is a big stretch and the next time a smaller one. We usually only do it twice. And you can visibly see his back is raised. Hope I explained it well enough.
We also switched our feed to a high fat low sugar feed that keeps our boys top line looking good.