Feral Cats in the Winter

Besides our two "indoor" cats, we have a feral cat who spends most of
his time in our back yard. Adopting another cat isn't in the cards for
us, though we do feed him regularly. He was skin and bones when he
first came around, but now looks healthy, despite being very skiddish
when we open the door to feed him. My question is how concerned should
I be about the winter weather and this cat? We live in Northern
California, where temps rarely dip below 30 degrees. We've taken a cat
carrier and lined it with towels to give him a little shelter at night,
though I've never seen him use it. Do feral cats need a hand in the
winter? Should we provide any other facilities for him, a blanket to
snuggle in, or an actual cat house of some sort?

-Fleemo