Wendy, unfortunately, doctors don't really know what curves will progress. They can give you the numbers, but that's it. It's anyone's guess, really. I was diagnosed when I was 9 with a 13* curve. It was watched pretty losely for three years and a year went by while the hospital backed up my check ups. That was when my curve suddenly went up to 43T/31L, against everything my doctor had predicted. So basically, spines will surprise you!
I was able to live a perfectly normal life until my curve progressed, which is great. I'm really thankful for that. And your son will be able to as well until treatment other than observation, if any, is needed.
I'm Karin. I had surgery for my scoliosis on October 13, 2005, and am doing well! I formerly wore a providence brace and did PT for treatment.