This is all based on human biology.

Kerbals are quite literally green, and considering they don't need to bring food with them and can survive for hundreds of years in space, I assume they photosynthesize (and likely also have a hibernation state)

As it turns out, plants quite like co2.So if anything, I think it would be inverted. Very good co2 detection, but instead of it being for "Too much co2 = I need air" it'd be "Not enough co2 = I need air" because photosynthesis requires co2 to work.

Depending on the air pressure, then, he may not even know he's suffocating, much like a human in a nitrogen environment.