California King Won't Eat

I have a 6 to 6.5 year old California King snake I received as a "rescue" snake 1.5 years ago. For the longest time he was eating happily and all was well, he ate one small rat/week. About 3.5 months ago he became very disinterested in eating, almost afraid of the food. He is about 5'4" long and weighed just over 1 lb. For almost 2 months I waited and without change took him to my vet who helped with an assist feed of one large mouse. He also received a vitamin complex shot to stimulate appetite but this did not work. Some eye caps were removed in case he couldnt see the food properly and this has not worked either. Nothing has changed for him as far as habitat, temperature, amount of handling, noise level, light during day or night or anything else. His one and only change is not eating and the small amount of weight he has lost, though he is not as light as when I first received him. Any ideas please? Could it be the fact that its winter?

it could very well be due to the time of year, many snakes refuse to eat. I have a Viper boa that hasn't touched his food in 2 months, I just keep offering it to him,in case he decides he is ready, if not my "garbage can" Buddy the northern pine snake eats it LOL