Based on this pic,
I think all it needs is smaller shoulder width and smaller armholes, that should take care of the extra material in the upper back & upper arm area, maybe take it in slightly at the midsection/navel area too
also idk why you guys are worried about it looking tight at the hem, isnt it going to have a 2-way front zip?
I would agree with taking material out of the back of the shoulders and the back. But the arm holes are really high. This dude is not at all big. Check that first picture, and look carefully to see how high the arm holes are. And he was wearing a tee. You take any more out of the armhole, and it will cut off dudes's circulation. Maybe we could slim the upper arm a bit as well, but unless you are an enemy of arms everywhere, don't tighten the arm hole any more.
fwiw, though @Thurston Bros is not a fashion guy, I am, and have seen a lot of clothes from many designers. And the thing is already slim through most of the body, even by "fashion" standards.