Recognize what's going on and encourage your partner to make positive change. Larry Cappel has 15 years of experience in couples counseling and is a licensed marriage and family therapist.
Let your partner respond, and then find a way to suggest that perhaps SAD is the culprit. Make it clear that you expect him to figure out how to do it for himself and for the family.
Be sure to reassure them that you don't think he is bad or defective, but that he may have a legitimate illness. Be encouraging and supportive, but don't continue to do this.
You have to step back before someone else will step forward.
There are three types of professional help to consider: One is to see a medical doctor to discuss medication, another is to see a trained psychotherapist to help relieve the underlying anxiety, and the third is to see a couples counselor to help with the issues SAD creates in the relationship.
Once you know what you're dealing with, you can take some skillful steps toward making things better for all of you. Create a time and space, in an agreeable setting, to talk with your partner about how you are feeling.