Teaching the horse to stretch through his back, engage his hind end and bring his hind legs under him on the lunge and under saddle, as well as stretching his neck out. And make sure they’re moving forward!!Once he begins to work through his back and doesn’t work hollow you could start lunging him over raised trot poles (one side raised). Keep in mind these suggestions are for if you do not have hilly terrain for turnout or work. Remember it takes one year to establish the top line in a horse and to build the muscle and two years to bring them into collection, you will not see heaps of improvement after a short while of work.
If you don’t have hills like me ... a lot stretching of the neck, with the nose of the horse closer to the floor. It helps the back moving up following with the abs.
Try to do a flexion of the neck on one side for couple of seconds and then let the horse tend his nose down, then change the side. Repeat until it becomes natural for him to stretch down when you longer your reins.
Going up and down a small hill is really helpful but a lot of horses don't like that so cavaletis and trot work are the best:)