I also would tend to downgrade ASD concerns based on the shifting of his obsessions, and how they correspond with new skill areas. Most children--gifted and otherwise--have periods of focus on mastery of a skill, or working through a developmental problem. It may be more pronounced in GT kids.
Our very clearly ASD DS12 has had shifting obsessions over the years, most of them academic. Sometimes they last a year, in which he quickly masters an enormous body of material at a high level, then he goes on to the next one.
What's distinctive about the ASD obsessiveness (again IME) is that it inhibits other kinds of learning and flexibility. That is, the child needs to keep talking about surface temperatures of planets when other kids are trying to engage him in play, or the teacher is saying it's time to clean up.
If the obsession is interfering with learning other things, with appropriate social interaction, or with the kind of flexibility that's needed to function in a variety of environments, that's where I would call it a problem rather than a gifted quirk.
YMMV, of course.
DeeDee
