I am so sorry to hear about what happened.
I have to admit I'm not sure if a sheriff's department is the same as a police department in aspects regarding criminal behavior. If there is a difference as I think there is, I would suggest going to the police and reporting the incident. Make sure you get a copy of the police report, and you get the name of the police officer who took your statement. I would also suggest getting a criminal attorney not residing in your county. This attorney can be in direct contact with the district attorney and most likely put pressure on him to file charges. You can also file a civil lawsuit against the attacker and have the attorney bring the charges in front a judge; it will be very easy for an attorney to request a change of venue because of the conflict of interest in that the attacker is the mayor's son.
I have to ask, only to be better answer your question; did he wear a condom? It's possible if he did that DNA would be inconclusive. While this is a setback in some respects (that you have no DNA to prove the attack), it is also a blessing in disguise in that it is less likely that you could have contracted something from the attacker.
Whatever you do, don't give up. You are absolutely right that he deserves to be punished. There are many people that are willing to help, me included, to see that justice is served. You are a very brave woman and your voice will be heard. If you need help finding an attorney, let me know. I will do my best to help you find one.