[GuyWithThePie]

Forget about his pants, what about his collar?



It even clips through his arms in official 3D artwork!

[Terrmedia]

it's supposed to rest on his shoulder/arms but that's just as impossible 

[Mobotropolis]

His color is held in place with the power of dreams!

... but seriously I guess it made more sense in earlier development when he was supposed to be tethered to some sort of floating monster-Huepow and they never changed it.

[Fennekin]

More jumpsuit theory support, now that I look at it. Goes along the lines of the Namco X Capcom artwork; look below the zipper.

[Terrmedia]

So really it can only be:

A: A jumpsuit

B: Pants/singlet shirt combo

interesting

[Derpy Hooves]

Umm...

His you know what doesnt exist.