It's good to cool garters down so they can truly get some sleep. If he is eating that is a good sign. Snakes hiding for long periods of time sounds like snakes just being snakes. One of my fox snakes stayed in its hide box for nearly 3 weeks. I just seen it out for the first time yesterday. He was perfectly fine. He will be getting a warm bath soon and put in a cold closet for the remainder of the winter.