I posted a similar question about scheduling for my 6yo DS earlier this year and got a great response from another mom who's been through some very creative scheduling. The child had to miss reading in his home grade to take math two grades up, so then went to a different class in his home grade for reading, and then back to his own class. So he basically was in three different classes throughout the day.
If your school is like ours though, all the classes in the same grade have lunch and recess at the same time and that probably wouldn't work. I can't imagine my DS having to miss that little bit of social and active time with his classmates during the day, especially since this year, his best friends are all in other classes and that's the main time he sees them.
It sounds like your main options, if you want to preserve some social/active time for him, are to take 6th grade GT math every other day - which may be possible if he already knows most of the 7th grade math curriculum as you said - or to take the 7th grade math class. Your instincts probably have a lot to say about which of those options is the best (sounds like they're already indicating 6th grade GT math). I think Grinity's advice is good because it allows your DS input.
We haven't gotten our own math schedule figured out yet for the year, so I may be posting for similar advice in a week or two.
