Why is he on the cool side?

I wouldn't worry about him being on the cool side, as long as you have identical hides, and the right temps he will be fine. Some snakes stay on the cool side all the time, some the warm. All bps behave differently, some like it hot, others cool. It's just their personal choice.

My snake, Mila, always stays on the cool side, it seems that she hates the warm side, she only goes over there when she wants out of the cage (which is right now) I even warmed the cool side up a few degrees & she got out of there as soon as it warmed up & laid out in the open in the middle of her cage. I've had her for around 7 years & she has always been this way.