i'm a total fucking dork.
i found this little dog store in SoCal here (vay-cay with the family, you know how that goes) and checked out what they had. one of the products was a little leather harness. hand-made in canada, so naturally i felt like picking one up. it was more expensive than i should have ever considered spending on something like that, but hey. it seems like it'll be useful on a dog that's got breathing issues anyway.
http://buddy-belts.com/m_37.asp
If he pulls on the leash even a little bit, take the harness off. Dogs that pull carts/etc wear harnesses. If you don't want to make the pulling worse, don't use a harness and use a choker collar, or something else. People generally buy harnesses because they think its 'safer'. Yes, it IS safer...when your dog pulls like a maniac, but it gives you no control over a dog at all, and generally encourages the pulling. Bully breeds have breathing issues, yes, but they could have the worst breathing issues in the world, and the fact of the matter is none of it should matter since when a leash is on, there should be no pressure on the neck if the dog walks properly.
If he walks nice, then more power to you, put on whatever you want. But from a training standpoint, there is zero need for a harness. They just aren't used, ever.
And this post wasn't meant to knock you down, and you definitely aren't a dork. There is a lot of small, up scale, original pet shops in Philly and I bought Plato a TON of shit from there when I got him. TONS. The money I spent is startling, lol. I was just offering advice in terms of the actual harness, being that I didn't know what your conception of it was. If you need any help, let me know.