Hi yuiu7 and welcome to the forum. I'm sure someone much more knowledgeable will come round later and be able to better help you.
Some more information may be helpful in the meantime.
Where are you located? As the weather gets colder/the days get shorter, most hammies will adjust their habits to reflect that. For example, all of my hammies have begun sleeping longer, getting up later and are less active than they were in the summer. If the weather is getting cold where you are, then this may explain his change in sleeping habits/activity level. His change in nest location may simply be that he found a more comfortable spot. I have a hammie who has a preferred sleeping spot when the weather's warm that he will completely ignore when the weather cools. Wheel-wise, he may be using it while you're asleep. When I'm concerned about whether a hammie has been using their wheel or not, I'll put some of their bedding on the wheel and check the next day to see whether or not it's still there/been disturbed.
How old is he (if you know)? Some (most? I'm not sure) become less active once they outgrow their baby months (well, years to them) and lose all that excess energy. I've noticed most, if not all, of my hammies have been collectively eating a little less than they were in the summer but they're also a lot less active.
When you say he is not eating
properly, do you mean he is just picking out some of the food and leaving the others? What are you feeding him and how much (it's possible he's got a hoard stashed in his cage that he's been picking food out of). If the problem is that he is only eating part of his food, picking out his favorites and leaving the rest, then you can try feeding him pellet food, which gets boring but will make sure he gets all the nutrients he needs. You can also try the tip(s) on this
video by ErinAnimals.
Has he been drinking and/or pooping normally (eating, drinking, and pooping normally are the three things I tend to first check/ask about whenever I'm concerned/curious about a hamster's health). If you know his weight from before his change in habit, then you can try checking to see if he's lost any weight. If he has lost weight or isn't drinking/pooping normally, then I would take him to the vet as that would certainly be concerning.
It he hasn't lost weight and is drinking and pooping normally, then I wouldn't be so concerned and put it down to his being a fussy eater (which would need to be "fixed" or helped to make sure he's getting all the nutrients he needs) and a change in weather.