IXL is a good program to ensure kids master topics but it requires a lot of work and only teaches if you miss the question. I think many kids lack the discipline required to learn a lot through IXL.
As Portia indicated, AoPS is probably the best program for mathy kids. Their site has so much information and so many tools. Alcumus, Mathcounts Trainer, the Forum, the classes...
If your kid really likes math and is interested in learning, perhaps you could buy the Intro to Number Theory or the Counting and Probability books from AoPS and have him work through those.
