Coming late to this, but I wonder whether, for people who don't just innately love numbers, learning mental maths strategies is just going to seem pointless until they've had plenty of experience with problems (real problems, I mean, not sums) that require them. It might well be best to come back to work on mental maths strategies a few years from now. There are really very few ideas in the first years of school maths. If your DS conceptually has them, I see no reason not to let him freewheel on until he finds something that's hard, interesting or preferably both, and then use the natural slowing down at that point to reinforce.
You might want to play with the balance between making him practice mental maths and allowing him not to think about that at all - IMO there's no harm in having a calculator to hand sometimes, just don't let him have it all the time!
And try to find something that's fun for him, whether it's Murderous Maths, sudoku, logic puzzles, or whatever...
