In all honesty... He is what I consider a "typical" AQHA/APHA cremello stallion. A horse which, on the surface, gives you some wow factor... But if you paint him chestnut faults you wouldn't "normally" breed to pop up.
I don't find his back too long, of itself, but what does lend a lot of concern for a riding horse is his long loin and worse still is the LS joint placement which seems to fall quite a bit behind his hip. All of this is taking strength away from the horse's back...He does have good loin girth depth... Which will help HIM counter those faults, He may not have trouble with this if he is trained and worked correctly, but isn't really something you want to reproduce though... And depending on the mare he is bred to may end up with a foal which is worse.
Then there is his hind limb contruction... Which is not uncommon within AQHA, especially some bloodlines... But he is a bit straight behind, granted seeing him at about 8 years old may show he grows into it some. Many growing horses can look straight at times and balance out... But some stay straight.
He does appear calf kneed.
So neither his front limbs nor his hinds are "ideal"... From a structural standpoint.
His overall balance is downhill... My gut says he will stay that way. Which, again, is not uncommon for a Quarterhorse but it will make it harder for him to truly round out and move correctly.
He has a lovely front end, lovely neck shape, and shoulder. His chest looks deep and wide. His overall balance is not bad. He has strong points, and obviously a great temperament... He is a lovely, eye appealing horse, But in all honesty he is not, IMO, truly breeding quality... His faults are what I would consider "major weaknesses" involving both his back and his legs, even if they aren't all "extremely" bad, they are there.
I believe the last stats I read said that AQHA registers around 130,000 foals every year... IMO it is time for many Qh breeders to get tougher on their breedingstock. (consider that registries like KWPN, with inspections, register something like 15-30,000)
The thing about breeding is you really roll the dice, especially with a virtually unproven animal, on which traits will be passed on and which won't. We all know that every horse has faults, and choosing breedingstock is really about deciding which faults can be overlooked, which get cancelled by which strengths, and which are "no no's" to produce offspring which will have long, sound lives.