With the English saddle, it will just depends on how it fits him. The front should be the same angle as just behind his shoulder, or a tad wider. The back panels should be about the same shape as his back. You'll just need a good pad to fill in for the middle
For western, I just don't see you finding a saddle that will work with him, even with corrective padding. His back is just so SHORT, and coupled with the sway, it will make fitting a nightmare... The Aussie will be your best bet, if it's got the padded underside, similar to an English saddle.