Far North, how much clearance do you have over the withers. It looks pretty close and you're not in the saddle. Check the back of the saddle for the end of the bars which shouldn't go past the last rib. I think you will have more of a problem of the saddle wanting to crawl forward. If so, this can make him sore in the shoulders. I also suspect with his back shape that there will be some bridging. If crawling occurs you might want to get a crupper. Someone will have to attach a large D ring to the back of the bottom of the cantle. The crupper will hold the saddle back.