I don't think you can stop his progress. You may keep him 2 years ahead instead of 4 but it still will be an issue. KWIM? I say if he likes it right now go for it. The Number Devil and Penrose are both great books. They are not workbooks but fun books about math. Get him workbooks IF that what he wants. We used to talk a lot about math at that age with DS6 (our math kid).
Get him logic puzzles, Sudoku, Mastermind. You may want to look into Critical Thinking materials. Let him watch Cyberchase. You may also look into math manipulatives if that's something he likes.
Have fun and don't worry about him being ahead. It's too late for that anyway
