try flexing their neck to the inside then to the outside
I suggest you to do some hill work with the horse's head low and his hindlegs engaged with impulsion. It will help strengthen his back. To work the horse's cardio the best mean is to have a long ride out with a lot of trot and canter.
If you can't go out, regular training sessions will help the cardio work.
You also need to always make you horse's back in the right position to work so he works his whole body when you train him.
Good luck :)
I think it would help to try and get the horse lower his neck and go in a slow trot, so that his back is working 100% and building those muscles💪🏼😊, It really helped.my horse to learn how to use her back correctly