First of all, this is not very rare in CAH males and a recent paper states that at least half of males with the condition will experience this problem at some stage.  It is caused by undertreatment of glucocorticoid as you know and if men/boys do not take their medication, this is one of the side effects.  In most cases however (70%) the adrenal rest tissue will shrink and the tumour disappear just by treating with a more appropriate dose of glucocorticoid.  Dex is the most potent and often used for more persistent tumours.  If unsuccessful, usually when the tumours have been present for many years (which is unlikely if your son is only 18), then surgery is required to remove the adrenal tissue (not the testes!).  There is no reason to think this will affect your sons future fertility as I said, this can be effectively treated. 

I personally feel all males with CAH should have a testicular scan at least every few years but it is only recently that doctors have realised how common the problem is.  My son is 15 and had his first scan earlier this year and thankfully there was no sign of tumours.