What you describe sounds familiar. Have you heard of Seasonal Affective Disorder? The following url is the National Mental Health Association's online fact sheet on SAD. (yes, I know it's an ironic acronym, but there it is.)
http://www.nmha.org/infoctr/factsheets/27.cfm
It's just a thought. If, however, your son does turn out to be at the beginnings of Depression (which now affects 1 in 5 teenagers, according to NMHA), there are things you can do to help him beyond medication. Be aware that in many teenagers, the medication typically prescribed for Depression only causes it to get worse.
The following are guidelines from the National Insitute of Mental Health (www.nimh.nih.gov) on how to help yourself if you're depressed:
* Set realistic goals in light of the depression and assume a reasonable amount of responsibility.
* Break large tasks into small ones, set some priorities, and do what you can as you can.
* Try to be with other people and to confide in someone; it is usually better than being alone and secretive.
* Participate in activities that may make you feel better.
* Mild exercise, going to a movie, a ballgame, or participating in religious, social, or other activities may help.
* Expect your mood to improve gradually, not immediately. Feeling better takes time.
* It is advisable to postpone important decisions until the depression has lifted. Discuss it with others who know you well and have a more objective view of your situation.
* People rarely "snap out of" a depression. But they can feel a little better day-by-day.
* Remember, positive thinking will replace the negative thinking that is part of the depression and will disappear as your depression responds to treatment.
* Let your family and friends help you.
You've obviously got a child who trusts you enough to come to you for help with the problem, and that's a tremendously good place to start. If it continues, talk with his doctor about the best treatment for your son.