Do you have a room thermometer in there? Since I got one for DC I have been more aware of how bad I am at judging temperature. Also hamsters are desert animals originally so it doesnt have to be super cold to send them into torpor. Failing a temperature problem, you may have a poorly hamster, so might be worth taking him to the vet. If he is ill they may be very limited in what they can do for him, but still worth getting a second opinion on whether it is just a warmth issue or something else going on.