It depends on the tree size. I have a medium tree Gen-x and it fits my QH very well. I plopped it on my TB the other day (who has been a broodmare for the past 6+ years, hence, she is seriously lacking in the back muscle department at the moment) and it is NOWHERE near fitting her. She has high withers - it was hitting them and way to wide for her.
I only put it on her for 30 sec just to see what I was starting with - after she's back in work she'll be getting her own saddle.
The Gen-x is GREAT though, if I can find an affordable one that fits her it would be in my barn in the blink of an eye. I've ridden dressage, evented, competed in jumpers, trail ridden in it. It has a pretty flat seat - great for drops on XC.
Can you take it out on a trial? Or trailer your horse over to the seller's place to see if it fits? Maybe the seller could bring it to you?
It's a medium tree width, so from what your saying it sounds like it might not be the saddle from him. He's starting to muscle up a bit but he hasn't been worked up until I started riding him so his shoulders are still a bit weak. And he needs a lot of room in the withers. Can't try it out unfourtunately, so the search continues!