My first thought is, he's only 5, so everything comes with a big caveat. I agree with others that there is so little conceptual content in the elementary school curriculum that it's pretty easy to plow through it at light speed if you simply aim to check all the boxes.
My personal sense is that it is not a bad thing to let him rush up to a level where it becomes more feasible to offer more challenge. It sounds, though, like he is a child who could benefit a lot from adding as much depth to his work as his grade level allows. Hard problems, whether in math or language arts, that require slowing down, thinking, analyzing, and especially struggle. Where there isn't a straightforward process to get the result, but rather many ways of getting there, and maybe more than one right answer. Where the person who is "the best" may be the one who works the hardest at it and produces a lot of wrong answers on the way, rather than the first one to check the box.
(Materials like competition math and AoPS are good for this; I know the Math Kangaroo starts at grade 1. I'm sure others on the board could suggest equivalent "stretch materials" in other subjects).
So I don't think he needs slowing down just for the sake of slowing down - the pace itself (especially at this age) is not the problem. But it does sound like depth would help him learn and grow, and not build a vision of education as a "to do" list of items to be struck off as quickly and with as little thought and effort as possible. Given that, I think it would be better to build the challenge into his regular school day rather than have it as an after school add-on.
