Dylan, it's possible he just prefers the cooler temperatures, but there are a couple other factors to consider.

1. Is the cool side darker? If you have a heat lamp on the warm side, he may just feel more secure on the cooler, darker side.

2. Are the hides on the cool and warm sides identical? Some snakes prefer one type of hide to another. If the one on the cool side suits him better (maybe it's lower or smaller), he'll may just stay there regardless.

3. Is the humidity level the same in both hides? If you use a heat lamp on the warm side, chances are the humidity on the cool side will be higher, and your snake may prefer that.