His pants come up to exactly the point where he is shot in episode 7 (which is high for modern pants and does make him look like a dope).

A freeze frame for reference. He punched at the wound several times during his first lightsaber fight with Rey.

It has been established that Chewbacca’s bowcaster does tremendous amounts of damage (it knocks Stormtroopers head over heels). It’s very possible that Kylo had to have part of his torso replaced with prosthetics in-between episode 7 and 8.

Or maybe these are just space pants. I guess we won’t know until episode 9 comes out!

[Source: pandawaffle on Imgur]