Okay, then.
So, for starters that explains why he's so skinny. And that would be his actual form - normal, real-world people can't simply 'flex' and get bigger. Certainly normal people can't just 'flex' and put on 400 lbs, so whether the manga presents it as a superpower or not, it's not 'pure human ability'.
Number two, regardless of his mass, if his respiratory system is majorly screwed up then he's not a fighter any more as a real world human. Might be able to throw a few strikes or whatever, but with a 'mostly destroyed' respiratory system he gasses out PDQ and that's that.
So given all of the above, probably better not to include him because as a real world fighter, he's a 7'3" 160 lbs guy who can't breathe properly.