Holiday/seasonal depression

My husband and I have been married for almost four years now. We've lived together since 2014. He used to live up north where it gets dark early and snows a lot/and cold. He goes through like a seasonal depression during the winter/Christmas months. We've since moved South where it doesn't get dark super early and doesn't snow or get near as cold as single digits or anything. But he still seems to feel this way, that's why I've been thinking this is a seasonal thing that will happen no matter where we live. I think he's been feeling like this at least since his late teens. Has anyone else delt with anything similar or like this? How do y'all help your S/O cope?