It's interesting, because I think dressage one puts in better position than AP (unless you have more bent in your knees then I do). Also A LOT has to do with the saddle itself. Both Stubben dressage saddles I tried pushed my legs forward as well as Centaur, while my Vienna helps to keep them under me (I think I'm uneven though, so may be part of the problem why it's hard for me to find one that helps my position).
I never tried dressage Collegiate, but the AP I had seemed to run wide (I had medium that was a good fit for mine, most of other brands require wide for them). In any case I hope one of them will fit your boy.
BTW, if you come across Thornhill by any chance I'd recommend to look into one: those run wide and nice saddles for the price.
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