Until he is fitter, I wouldn't even try to buy a 'good' saddle for him. I'd buy the cheapest, best fitting one I could get and make up the difference with a pad and shims if needed. He's going to change so much over the next year that you probably won't be wanting the saddle you buy today at that point. So a treeless, if you can find one second hand, might be your best answer for the short term. If you're going to be doing dressage, I don't think you'll want one for the long haul but buying a Patrick or some other brand that's fairly pricey but fits, would be a save up for it project.