In 3rd and 4th grade we started doing some local math competitions. In the AoPS pre algebra book it kind of goes that way. They use some problems from previous math counts and AMC 8 competitions. If he likes grinding them out he will like these. I used to print out some of the competitions and cut single problems out and tape them to his binders. I had him go over them and try to solve them and then we would go over the solutions sometime that week. It was different than school math.
Sometimes it is not so much just acceleration when it comes to math but doing problem solving problems. You can buy a lot of time by just doing harder or different problems. He will still be sharpening his addition, multiplication and such but understanding how it all links together.
There are more problems with solutions out there then you will ever use.
This is just what we did in 4th to get through.
