In undertreatment, it was my understanding that testosterone is elevated and this causes the testicles to shutdown and sperm production is lowered or stopped?  Here is a PDF file that has been turned into html and if you scroll to the bottom of that page it discusses issues for men with CAH.

 http://216.239.33.100/search?q=cache:cu4iVTa-XgIC:www.endocrinology.org.uk/pdf%2520box/cah.pdf+fertility+in+males+congenital+adrenal+hyperplasia+dr+gerard+conway&hl=en&ie=UTF-8