The dream: I somehow met this family who had one child (about age 11 or 12) with mild autism, in summer sometime. They asked me if I would like to basically hang out with him and help him with everyday things like cooking when they were away. He was very nice, and in the dream I think I had known him when I was very little (about three or four) even though I had never known someone with autism at that age. We got to be very close. I think. I don’t remember much, but I remember one scene where his family and I were in their attic, looking out at the snowy forest (there was a big window). The attic was somewhat cluttered but very well furnished (like a living room would be). There was a fireplace. We were planning to go skiing.
It's difficult to say what your dream means without knowing what you mean by "autism" - autistic children are rarely described as "very nice" or able to become "very close" to anyone. The attic is most likely an A-frame chalet or cabin, most likely a remnant of an old experience from when you were "very little (about three or four)". There's nothing to identify the "one child (about age 11 or 12)" except his "mild autism", which I suspect is a pun - "aud-ism" to suggest a good listener (which would better account for the affection you felt for him and he for you in your dream), or maybe "odd-ism" to suggest an inoffensively peculiar individual.