I think the Singapore "Intensive Practice" worbook would be what I'd recommend. Someone (Texas Summer, I think, with my apologies if I'm not giving credit where it's due!) talked about the two-prong math approach: critical thinking and "tools" (arithmetical skills, checking work, etc.) That's been useful for me to think about what my DS needs, so I'll use it here to explain why I think "IP" is better for your DS's needs.
"Intensive Practice" is big on the critical thinking, and the problems are really tough in that respect. You have to have the "tools," too, but the critical thinking stuff is really challenging. Far more than the "Challenging Word Problems" books, I think. It's more puzzle-type stuff, brain-teasers, like you might do for fun. It seems like it's right up your DS's alley!
I can't speak to the other resources you're considering, but I'd say "IP" would be better for your purposes than "CWP" if you're thinking Singapore Math...
