Lauren -
My son, Dylan, was also diagnosed at a very young age (2 months) with a curve of 40 degrees. We believed his scoliosis to be idiopathic at first, but as he grew it became apparent he had some fused vertebrae. But, even after surgery for that (at 6 months old), his back is curving and we don't know why now. So I consider him congenital/idiopathic.
It is great news that the brace is controlling the curve! I know it is hard to deal with a health problem with a child. As parents we want our children to have a trouble/worry-free childhood. Try to be reassured by the brace working and not worry about the future. It bothers me that your doctor told you to expect it to worsen. I'm not saying they should leave you in the dark, but it doesn't help your family to worry about the "maybes." If the curve isn't worsening now, that's great! We try to take it one day at a time, focus on how well Dylan is doing now (sometimes it's not easy). But, it makes things easier for us and Dylan.
Dylan is 3 now and eventhough he has been through more than most adults, he has a great attitude. He doesn't see himself as any different from any other 3 year old. When another child points to his brace and asks "what's that?" he just matter of factly states, "my brace." When they ask why he wears it, he tells them because he had surgery on his back, like it is the most normal thing in the world. He doesn't know any other life, so he is happy. The same will be true for your son.
Take care.
Penny
Penny
Mom of Dylan, 3 years old, 4 surgeries, diagnosed at 2 months