You should contact an attorney in your area that does speeding ticket defense. They will be able to help you with your situation and give you advise.
A summons must advise a person of the charged offense and court date. Despite the additional language written by the officer, the listed code section generally trumps. Thus, your son has been advised, pursuant to the listed code section (46.2-870), that he has been charged with violating the law by speeding 64/55. This sounds more like a clerical error by someone inputting information in the clerk's office and not a legal technicality that would likely cause the charge to be dismised.
Darwyn L. Easley
The summons itself is not evidence in Virginia and the description is adequate to put your son on notice that he is accused of failing to obey the speed limit under Virginia Code section 46.2-870.
Code Section 46.2-830 is failure to obey a highway sign, which is different offense.
Both charges as written are 3 point violations according to the DMV. The maximum fine for both is $250.00. You can usually pre-pay the fine for less than that amount. A lawyer may be able to negotiate a resolution with no points. However, if your son is under 18, multiple traffic infractions may result in a license suspension.
Talk to an attorney that practices in Fauquier on a regular basis if you need to pursue defending this further.