I agree. Stretching is a great way as well as spiraling. I also do bending exercises on my older boy and that has seemed to help with him. Also I use a magnetic roller grooming mitt before and after rides to help loosen his muscle and have noticed a difference with his suppleness
I would do lots of shapes serpentines are great for suppleness and also spiralling in and out of a circle 😃
Stretching! I use a carrot and have mine stretch her head side to side and down between her front legs.