Try some carrot stretches between their legs and slightly pull on their tail which will make them raise their back which will as result make them stretch
Some really good stretches that I recommend using after every ride are stretching to the sides and between the legs. You use a treat and move it to one of the sides (around the stomach of hip) so that the horse bends his neck to get it. The other is done in the same way but by putting the treat between his front legs. Once the horse gains more confidence and flexibility, he'll learn to shift his weight and bend with his whole body, not just his neck.
Hope those help ☺️
Tickle your horse on the stomach 🙈
The easiest is to train the horse to go long and low under saddle.