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They don't try to go into hibernation, if they get too cold then their bodies begin to shut down & they go into a state known as torpor, that's a serious problem & if they don't get warmed up quickly they can die.
What is the temperature in the room where his cage is? If you're not sure get a thermometer & check, it should be around 18-20C both day & night.
If the room is warm enough it does sound as though he may be ill & you would need to get your vet to check him to be sure what the problem may be, if the room is just a bit too cold then that could cause him to just stay in bed more & not want to come out.
If the heating is a problem then you can try using a heatpad like a snugglesafe under but not touching the cage, it needs to be a few inches below the base, I always put a fleece blanket under my hams cages in winter to retain heat too.
A small oil filled radiator can be cheap to run but effective in keeping the temperature up too.