Don't ignore the ticket. You should spend a little money on an attorney to help your son with the ticket. As it is three years old, a dismissal of the charge is very likely. But do not ignore it and allow a warrant to be issued for not appearing in court. The county did drop the ball but the ticket must still be resolved.
You will need a traffic ticket attorney to help you evaluate the situation and determine whether the statute of limitations has elapsed. The traffic statute involved is: Texas Transportation Code § 542.404. Fine for Offense in Construction or Maintenance Work Zone, which states:
(a) If an offense under this subtitle, other than an offense under Chapter 548 or 552 or Section 545.412 or 545.413, is committed in a construction or maintenance work zone when workers are present and any written notice to appear issued for the offense states on its face that workers were present when the offense was committed:
(1) the minimum fine applicable to the offense is twice the minimum fine that would be applicable to the offense if it were committed outside a construction or maintenance work zone; and
(2) the maximum fine applicable to the offense is twice the maximum fine that would be applicable to the offense if it were committed outside a construction or maintenance work zone.
(b) In this section, "construction or maintenance work zone" has the meaning assigned by Section 472.022.
A case interpreting this statute has the following blurb, which indicates the court's sensitivity to the purpose of the statute (protecting workers): Evidence was legally sufficient to support doubling of fine for speeding in construction zone with workers present, despite defendant's argument that no workers were in the particular section of the construction zone in which he traveled and his offer of photograph of zone as proof there were no workers present; fine enhancement statute did not require workers to be in a zone of potential danger or to be visible, citing officer and defendant testified defendant was exceeding posted speed limit and that he was driving through a posted construction zone when pulled over, officer testified workers were present, and defendant's photograph showed only one directional perspective of the zone. Chance v. State (App. 14 Dist. 2008) 2008 WL 4390637, petition for discretionary review refused, rehearing on petition for discretionary review denied.
Your son being absent from the state may toll the statute of limitations, so you and he need to be careful, and to pay due attention to this situation.