I'm a teacher and started using ixl.com for math with my students as part of their math program (review) and they love it. My students have completed over 30,000 math problems in under 2 months. All I ask is they practice math facts 2-3x a wk on ixl. My students love the Explanation button that explains why the answer is the answer. I have many students who use this to teach themselves new concepts. Many of my students are exploring areas of math I have not introduced just yet and are using the button to learn the new skills on their own. Works for me.
On ixl.com I can see what each student is doing (problems and answers), they earn virtual rewards (like an old fashion sticker chart, but virtual)for correct answers, time on task, and skill mastery. It's an attractive site.
I run an after school program for learning fast facts and my students in this program LOVE ixl.com as well.
The skills are broken down by grade level(very accurate about what is typically taught in each grade). My students are practicing all over the grade levels. To master fast facts, they are using Addition and Subtraction practice in first and second grade. ixl.com is going to change the grade levels out to colors, but it hasn't happened yet. (The director of the program is an alumn of the school I teach in and we frequently talk about ways to improve the site.)
Have your son start out with practice in a grade or two below, just to get use to the site and feel good (earning medals and awards). If you want a list of progressive skills to work on ixl.com for fact mastery, email me and I'll send you the list we are using for school.
You might be able to get a 30 day free trial.
Good luck,
Stephanie