I don't think this whole indoors/outdoors thing is all that big of deal, but a lot of people seem to, so a couple of ideas: It appears that Dylan enjoyed doing outdoor stuff with his friends -- swimming, skate park, goofing around. Having spent some time in Colorado, with people who always seemed to be doing technical climbing, backcountry camping, whitewater rafting, I think maybe the question is one of degrees. When his mom said he's not outdoorsy (paraphrasing) maybe she meant that he's not into some of the more adventurous types of activities; he does more of the casual type of thing.
Secondly, I was thinking about this in terms of myself. On occasion I am forced to visit a place that makes me feel uncomfortable. While there, I tend to take long walks, spend hours at the lake, etc. Anything to stay out of the house and avoid the tension. Perhaps when Dylan was at his mom's, he liked being inside and around the family, playing games, talking, etc.; he felt comfortable. Maybe at his dad's he spend more time outdoors, to avoid interaction if there was some difficulty in the relationship?
