It seems odd for him to skip a grade and not be in GT classes. If I had the choice between a grade acceleration without GT classes or no acceleration with GT classes, I'd choose the latter. It seems a GT child would be better served by the depth/complexity and "mental mates" of GT classes.
If you decide to have him repeat 5th, how will you make sure he faces some challenges? Will the GT classes take care of this or will he need some enrichment outside of school?
I'm also curious about how he would feel about repeating 5th. I skipped and think I would have been horrified at the thought of repeating a grade.