Dear C.:
I am familiar with Math-U-See. My husband and I first saw MUS demonstrated by its developer. I am a very SLOW person when it comes to math, while my husband is a natural and accomplished mathematician. During the demonstration, Jim and I kept turning to each other and saying, "No WAY!!!" Jim said it because he realized this method taught math the way he "got it" in his head and I said it because I was finally "getting it"!
We have been using MUS ever since.
Most of the families in our homeschool support group use Math-U-See. Those children who use it are excelling. I know of two children who used it all the way until eighth grade and they had strong foundations for advanced mathematics and are very well-equipped for any college math or science requirements in the future. (Just so you know, MUS does offer advanced mathematics courses as well.)
I heartily recommend MUS to you for your son.
Best wishes,
M.
PS: Here's a link to my friend's blog. In this particular case, she discusses using MUS when her children were small.
http://www.homeschoolblogger.com/jaminacema/389991
Dig around on her blog for other entries. http://www.homeschoolblogger.com/jaminacema
She discusses this program quite a bit.