Treatments for Seasonal Affective Disorder?

My 24 year old, live in boyfriend gets depressed evey year around the same time because of the changing seasons. It's become so severe that it is affecting our once loving, devoted relationship. However, this time seems different. His depression is tearing me apart, and I don't know what I can do for him. Last night he told me that nothing makes him happy anymore; not even our relationship. He said he feels trapped in his rut.

This is a recent development (the past month and a half or so), and last week we had sunshine and temperatures in the 70s. His mood was considerably better during that week.

I have read up on light therapy and I think I will end up getting him one. At this point though, I need to know how to handle this. I don't know what I can tell him to reassure him that everything is going to be okay. How do I handle him?