Actually, a study of the history of math would require that he already understand a fair amount of algebra. I would strong recommend against a study of math history, at least until he is further along, or he will very likely not be able to follow the discussions.

He might enjoy books about math, such as books authored by Theoni Pappas or Ivars Peterson (although Peterson's books might be a bit advanced).

I do not mean to discourage you or your son -- far from it -- but algebra is the toolbox that is used in every other part of mathematics. In order to discourse on math, your son will need the grounding he will get from learning algebra first. However, once he has covered algebra (and a little trig and calculus will help, too), he will find broad vistas opened up to him.