You're set-up sounds really good. You mentioned brumation. Could it be that he is trying to brumate but because the temps are higher his body is still in an active state but he is not eating and thus losing weight? Is his behavior like it was last year at this time? What if anything is different? I have to hibernate my box turtle as when that time comes even though his temps are no different he refuses to eat. If I don't hibernate him he would die before spring.

Can you post a pic of him?

Linda