What you think is conformation is actually posture!!
I quickly had a look at your horse for you...
What may surprise you and others reading my post is many conformational issues like sickle hocked, long backed, croup high, over the shoulder etc. are actually not "conformational issues due to breeding at all BUT postural and can change!!
He is just a horse that is out of muscular /skeletal balance and it gives him the appearance of being long backed due to how is standing.
See we have only had horses that were out of balance through history to reference to - they were out of balance due to the fact they were used in wars and work and had many accidents and injuries! So due there injuries they would stand to make themselves comfortable... legs out behind, front legs underneath etc!!!
A few things to help you and your horse...If you massage his hind end or gluteals muscles this will help to break up the pattern! Also, if you push on your horse's ribs the most like feel tight- when we push on our horses ribs the should give to the contact as designed.....
Good luck and let me know if you have any questions!