With the added info, if you are on a budget I would opt for an Aussie until he fills back out. My boy started out in an Aussie, feels like a really deep western saddle. You can get a relatively inexpensive used/new aussie and use it until the shoulders develop. I just recently discovered that once your horses back shape changes with exercise (bound to happen) with an Aussie, you don't have to get rid of it. You can 'Awl' the saddle to fit the shape of the horse. I did it for my boy and he rides better than ever!