I think one needs to know both the mental age and the level of mathematical ability if one is to source a puzzle book. A seven year old in 'normal' maths development or a seven year old who does algebra / calculus / maps members of the Julia set when he's doodling or what have you?

My 8 yo nephew has some Lego games, one with sheep and one about pharoahs (Amazon http://amzn.to/iN8Kkh [aff]). The rules encourage modification and, for example the Ramses Curse one can allow for some complex payoff considerations.

... or chess?

Edit: Saw these Maths (and other subject) puzzle books, http://amzn.to/lUSxXp [aff] might be worth a look?