Hmm... honestly? here's my suggestion. Also, i can't believe i didn't think of this before (As i have on that would work perfectly.)
Find an OLD wet-suit that fits...
I'm talkin like something from the seventies (Go to a thrift store even)
The whole look of the MC's undersuit depends on it conforming to his body. So if you're forming a pattern like that... Use what is best suited. A wetsuit! And go from there. You can find old ones for CHEAP!
Not to mention, if you indeed, DO use pleather... remember that it's elastic properties are actually very similar to a wetsuits'. So when you form your pattern you should actually come quite close

(heck, even the zipper placement is just about perfect!)
I've been putting a LOT of thought into the construction of this. Once I draw up a final design i'll post it here.