Have you tried Sudoku? My DS now 7 is much the same as yours "math happens in my head, not on paper". Give him books with math games and puzzles, brain teasers etc. Just leave them out on a table somewhere. I would second looking into the possibility of undoing his hold and having him moved to a more appropriate grade.

Shari
Mom to DS 10, DS 11, DS 13
Ability doesn't make us, Choices do!