You don't particularly need to wet-form leather when it's that light. If you form it while wet, it's just gonna shrink up on him and then he won't be able to breathe.
It should be enough to work out the pattern using a piece of non-stretchy fabric. Whether he has the patience for this or not is mostly up to him. It's probably faster to get a single quick pattern off him than it is to make a duct tape dressform, just try not to stick him with any pins.
Transfer the fabric pattern to the leather (avoiding flaws on the leather can be a bit tricky during this layout), cut the pattern and sew any seams. You butt the seams against eachother, so no need for seam allowances or sewing it inside out. Once together, test fit it and tweak it smaller as needed.
I can't quite tell from these pictures how it fastens together; I would assume some combination of lacing and hook-and-eye fasteners.
You've got a lot of tooling to look forward to. Have fun with that. Careful using the swivel knife with leather that light. If you cut too deep, you can weaken it or slice straight through.