[Aegis]

Hey, so sorry your hamster is feeling this way. The only thing I can think of is that maybe he's trying to hibernate, as you mentioned cooler weather. Not all hamsters do this, only some of them, and it's normal and will go away if the weather is warmer. His instincts might be to sleep a lot when it gets cold out.
I know this is a late reply, has anything else happened? Is he doing better now?
Best of luck,
-Aegis