What you say to your son is: "Son, when you were born, you were circumcised and there were some problems that happened during and after the procedure. It is possible that you will need some touch up surgery to correct those problems. There is also the possibility that you will have some pain or other problems. If you do, you need to let me know so we can get them taken care of as soon as possible. Don't think you just need to accept it or that you need to hide it. I'm here with you all of the way and want to be able to help you. I can't know that you need help if you don't tell me."
He will probably have some questions at some time. Answer them as openly as you possibly can. Sons have an intense desire to love their mothers regardless of what has happened. Trust me on this one!
Now, to the other part of this. I'm not a lawyer so don't hold my feet to the fire on this but I believe you have some options. First of all, I believe you have some legal recourse and please call Steven Svoboda at Attorneys For The Rights Of The Child for confirmation and details. While the statute of limitations has expired for you to sue, I think you can sue on behalf of your child. The statute of limitations do not usually run out for him until 3 years after his 18th birthday. While he can not sue on his own since he is a minor, I believe the legal guardian (you) can bring legal action on his behalf for damages he has suffered. One thing you must do is get copies of all of his medical records. This should be done without telling the doctors or hospital why you are doing this. It is not unusual for medical records to become mysteriously lost when there is a pending lawsuit.
I would recommend taking immediate action on this and you can confirm it with Steven. Even though there may be no current sexual disfunction, there is the spectre of it in the future. There is also most likely surgery in the future and there is the disfigurement that your son will bear for life. I suspect that is enough to bring suit and if you win, your sons medical bills will be covered as well as compensation for the disfigurement, pain and suffering and other compensatory damages.
The reason to go ahead with the legal action now is because the doctor may be dead by the time your son is 18 or he may have declared bankruptcy. It is very difficult or impossible to collect damages from an estate especially if if has been settled. It can also be very difficult to collect a judgement from someone who has declared bankruptcy especially if the reason for the suit pre-dates the bankruptcy. The hospital could be sold to a corporation and could be sold several times making collecting from them difficult and after 18 years or more, few if any of the people involved will still be around and memories will be even more clouded. There is also the possibility that the doctor was a part of a professional corporation or a limited liability corporation. If that is the case, you would be suing the corporation, not the doctor. If the corporation is disolved, there is nothing to go after. However, it is possible to separate the doctor from the corporation and sue him as an individual, it's just much, much more difficult and not always possible As a matter of fact, in some states, it is almost impossible!
Good luck to you and your son and keep us up to date on what is happening.