Hmm, without pictures it's hard to say. Does he have high withers?
Something to think about......Big Horn makes a gaited horse saddle that has fit a lot of horses I've tried them on (I used to work in a tack shop before I moved back in December and I did a lot of saddle fittings) so maybe that's an option if your current saddle doesn't work out. They are actually relatively inexpensive, it's a half synthetic saddle but the best quality synthetic type saddle you can find. Really the only difference in that gaited saddle is that the bars are flared to allow for more shoulder action and movement, but I like that quality for ANY horse. It comes in a standard or wide bar.