I have a healthy male HTCT leopard gecko. He is in a 24" x 24" x 8" cage with two females. There is a shed box and three hiding spots. The last week he has stopped opening his eyes unless I provoke him to do so by sticking a cricket near his mouth. When he does open them they seem alright, perhaps a little sunken. His weight is alright and his behavior is fine, I am confused. What could cause this to happen?

