I don't understand this at all. I appreciate what abetadad is saying, but as far as I can see, it just doesn't relate to Cobblers situation.
Cobblers would explain to her DS that most people conceive through sex, but that they were unable to, and so had their DS via IVF. If the DS ever asked what the problem was - which I think is unlikely - then the honest answer is "I don't know". It was unexplained infertility. I don't see how the single testicle would ever be mentioned.
I have a personal interest in this, as our infertility was very explainable as DH has no sperm at all. Even ICSI was out for us. So we used a donor. We haven't been open about our infertility at all with most family and friends, let alone the reason behind it, partly as we both know that male factor infertility is still a bit of a sniggery subject to alot of people. And some idiots still equate fertility and virility. But when our DCs are older and start asking questions, we will be honest and open with them. We know that this means that it might suddenly become a very public subject - our DCs will find out at quite a young age and might choose to stand up in front of their classroom and announce it! But we are willing to take that risk, because for us it is really important that we don't lie to our children about their origins. There is a lot of research that backs up what we plan to do - children who are lied to often find out in unexpected ways, and are understandably hurt and bitter. Children who find out naturally through the normal process of asking questions are generally very comfortable about it.
Perhaps it is different for you, as your DH is the genetic father, as well as the actual one, but it is something that I feel quite strongly about. Just my 2p.