He might have seasonal affective disorder. This is where a lack of sunlight can make you down. I'm not saying that's 100% of his depression, but it could be making the holidays even harder (and a lot of people find the holidays to be difficult; he is far from alone there).
I get SAD and I have a small full-spectrum light which I can plug into my USB port. I can't recall how much it was but I am thinking under $20. It was from Amazon. It also helps me to both get physical exercise and to get outside, even if it's cloudy out, so long as it's not actively stormy. 10 degrees F is horribly cold but if it's not snowing, I'll often go out in that.
To combine them, I go out walking, and in cold weather that's practically a necessity to try to stay warmer. I have texting gloves and I do the whole nine yards when it's really cold, as in long underwear, two pairs of socks, insulated boots with liners and spikes for ice, jeans, turtleneck, sweater over that, a pair of scarves, heavy parka with liner, texting gloves, and hat. I look like a blonde Eskimo but I stay warm.
I'll also shovel snow if it's there, as that gets me outside and working out, plus it's practical.
I'm not saying this will cure everything but any of those things should be good for him.
I know you'd love for him to have a girlfriend and I respect your thoughts but I am also seeing it from a future prospective girlfriend's perspective. If he depends on her for his good moods, what'll happen if she leaves? Or if she wants to, and his condition makes her feel guilty and stay, even when it's not the best thing for either of them? A romantic connection should not be responsible for his well-being in quite that way.