Check out the CTY website - they have math all the way down to pre-K.... if they get computer time as part of the K program, it might work well. Again, I know nothing about it, but I am thinking of trying it out with my dd. You have a lot of lead time, so perhaps he can try one course now at home so he can get familiar with the website and test out what level math might work for him prior to September. At least this way he will not end up with a two building problem down the line.
http://cty.jhu.edu/ctyonline/courses.html
