I know you say that he won't eat outside the enclosure, but it depends also on your approach. You can try taking him out and putting him together with some food in a plastic box or so. Make sure it's dark where you put him and don't disturb him. Let him stay there for the whole night and check in the morning. This has to be in a room which has at least 20°C.
But first, James idea is a good one.
