Don't waste money on resin, plaster is WAY cheaper. I have done this. Frankly unless this dude is a singer or does yoga, you will have issues around the lower rib portions while the plaster sets up. Most ordinary people breathe with their whole rib cage, trained singers and practicers of yoga learn to breathe through their diaphragm and thus can keep their ribs still while the plaster dries. "belly breathing" it is commonly called. It isn't hard, but a little forewarning for the one getting casted helps.
Let the one getting casted that plaster gets hot as it cures! It shouldn't get hot enough to cause burns, but it does get warm enough to make people nervous. I have casted and been casted myself multiple times. It's messy, but kinda fun too.
Use the gauze strips to get the fine nuances of shape. Then once you are happy with that, get your pure plaster to almost-too-soft-for-cookie-dough but thicker-than-cake-batter consistency, slather on an inch thick layer of it & wait for it to set up. For an upper body you will need to do the process twice, front & back. The hardest part is making sure the castee is in the same body position both times. It's harder to do the plaster with them sitting upright, but it's more consistent in body position.
Once your casts are pulled & fully set, paint the inside with a varnish, a LOT of varnish, to completely seal the plaster. Now you can trim up the sides, strap the front & back together and pour a new, very wet slushy, batch of plaster inside to get a proper mold. Now you have an exact replica of the torso to work on.
Is it a lot of work? yes, three days worth at least before a single element of the actual costume gets touched. But it's worth it for really high quality work.