A negligent driving in the second degree infraction is one of the most serious driving infractions you can receive, and can have serious repercussions with your insurance company. It would likely be worthwhile having a consultation with a local infraction attorney to discuss the facts in more detail to see what your options are. A small cost at this point in time to retain an experienced attorney to help you through the process may pay off in the long term given the insurance premiums will likely go up quite a bit if the Neg 2 goes on your son's record.
Jeff Holmes - Attorney at Law - www.vancouverdefense.com - 360.975.9288. Disclaimer: This answer does not constitute legal advice. This information is based on general principles of law, as well as my general experience that may or may not relate to your specific situation. This information is not meant to take the place of actually consulting an Attorney in your jurisdiction. If you would like legal advice, I would recommend consulting an attorney in your locale.
I agree with Mr. Holmes. The only things I would add is that it is even more important to fight it because of his age. The insurance consequences will be enormous and it may put him in a position to having his license suspended should he get anymore tickets. Negligent driving based on an accident is hard to prove since there usually aren't any actual witnesses to the driving.