Riding/saddle position

The problem with a downhill horse is that the saddle will crawl forward, jamming into the scapula or even up onto the scapula. A saddle should never ride on the scapula. A back such as this usually requires either a crupper or breeching to hold it in place. Too much padding to raise the pommel or forks will require a tighter cinch to prevent the saddle from floating. This too makes for an uncomfortable horse.

Trouble is we are all guessing at the problem, without good clear pics it is impossible to say what the issue is, even WITH good clear pics it can be difficult

^^Exactly. At least one good picture from the side and a video of him being ridden would help a ton.