I recommend your son go and meet with a few criminal defense attorneys here in Tulsa that have spent some time in the trenches. Although a speeding ticket does not appear on its face to be an overly complicated matter, there are a number of factors at play.
First and foremost, the City Prosecutor doesn't feel they have to "work" with an unrepresented defendant. It isn't until the defendant hires an attorney that they feel any real heat. Up to this point, the Prosecutor has felt they have your son over a barrel and can dictate the terms. Hiring an attorney changes the playing field substantially. It is a real game changer. Now all of the sudden, a traffic ticket becomes work for them, and the fact of the matter is, traffic tickets are money makers for the city. They aren't supposed to be work and take up the time and energy of the City Attorney's Office, not with the volume they handle.
Second, a seasoned attorney can usually look at the ticket without even talking with the client and quickly determine if there are any available defenses. They are looking for the name of the officer (we usually know most of them), their badge number (how long they have been a police officer), their assignment, time of day, method of determining speed, location of offense, etc. The review of the ticket and a brief and confidential discussion with your son will usually be enough for us to determine the appropriate course of action.
Third, the prior driving history is always at issue. The prior driving offense, "only 9 over," usually cues us up (and the prosecutor) to believe, and sometimes wrongly so, that the prior offense was actually a higher speeding offense that was reduced down to 9 over either by the officer that wrote the ticket or prosecutor that pled the case out. Regardless of the truth of this assumption, this issue has to be addressed when dealing with the prosecutor.
Needless to say, a jury trial and its associated fee (it usually costs you additional $$ to empanel a jury) is certainly available to your son at this point. However, an attorney will be able to best advise your son if it is in fact the best available option. It very well might be easier and more effective to hire an attorney to enter an appearance and communicate with the prosecutor "lawyer to lawyer" and attempt to get your son a better deal. I know that during my time as a prosecutor, I always was more receptive to argument of counsel than that of a defendant.
The prosecutor is correct - your son should get a lawyer! An ethical prosecutor will want a defendant to be represented by counsel. In criminal cases, there is an obligation for the accused to be given an opportunity to obtain counsel, and if the accused cannot afford a lawyer, a lawyer should be appointed. Your son may or may not qualify for a public defender, and even if he does qualify, sometimes private counsel is a better choice.
Many jurisdictions no longer do jury trials for speeding tickets, and other jurisdictions only do jury trials in cases where the speed is really high. I am not an attorney in Oklahoma, but I suggest that you help your son in arranging meetings with some Tulsa attorneys so that your son can begin to get representation and so that he can start to mount a defense. A prosecutor is more wiling to listen to attorneys who present a good defense than a person who represents oneself.
A speeding ticket is something that can be fought and won if you have an attorney who knows how to challenge the machines that calculate the speed a person is going. None of these machines is perfect. While it is true that in some circumstances a ticket is a money-maker for the state, in your son's case a prosecutor could just as easily look at your son as a major accident waiting to happen and decide to "teach him a lesson" and aggressively prosecute him. Your son may be viewed as an 18-year-old kid who has a lead foot and could easily wind up killing someone if he isn't taught some respect for the law. Your son needs a lawyer.