>>56254339A blue pijama. He is very conservative in his tastes so he only sleeps in his comfy blue pjs, or grabs an old white shirt and some old jog pants, and sleep with that.
Side note: I wanted to write how he accidentally got a girl crushing on him when he trained in Sinnoh. Camping in the mountain is cool, but he made it extreme by just carrying very little survival things, and no tent, so he made igloos. Turns out is really hot inside, so he slept in nothing but his tightly whites. And who else was with him? A skiing rescue woman who was sent to search for him because people believed he was dead. 