I would also go with the first doctors advice, but I would also look into a genetic test that way you would be sure.  We just had genetic testing done on our son and myself and husband because I am preg. with our second child and in order to diagnose the child thru amnio we had to determine the exact gene and all.  You would know for sure whether your son actually has CAH or not.  I do know that it is pretty common for levels to be off with premature babies.  I had a friend who had a baby just 6 weeks early and they had to rescreen her a few times to check levels, but she did not have CAH.  If your son does have CAH he can have a great life.  Our four year old is just like every other four year old and most people have no idea he has anything wrong.  Best of luck.

Carrie