Depending on how built he is, I'm with verdatum - you may not need to wet-form to get that chest curve. Wetforming something that large, it won't just be drying for hours, more like days, and I don't think he's going to sit still that long.
If you really WANT to do it, for added idk badassery (so that no matter how thin or thick or built or not the body inside the armor is, it will still have a curve at the chest to imply nice pecs underneath), you need a form that is pretty much the same size and shape as him. So the plaster body cast would be one of few options. It needs to be something sturdy, so foam or other quicker, cheaper options are right out.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com