There are three primary ways to achieve that goal:
1) Post bail. Once bail is posted, the warrant is quashed and a new court date set;
2) Contact the court and determine their warrant quashing procedures. Many courts have a procedure to quash warrants by scheduling an appearance with e clerks; or
3) Negotiate with the prosecutors to present an agreed motion to quash the warrant.
Under any of these options, it is imperative you retain experienced legal counsel to assist him in this matter.
You really need to heed Mr. Nahajski's advice. Your son will need an attorney to represent him through this entire process. You should retain an attorney to assist your son in getting his warrant quashed.
Most courts have a specific procedure, along with a separate calendar for quashing warrants. You can find this information out either by calling the court, or checking their web site, typically. If the information is not readily available, most court clerks can give you the specifics in person. Most of the time, quashing a warrant can be done without having an attorney present. However, as the previous answers have suggested, it is especially helpful to have an attorney present to make sure that nothing goes awry. If your son is suffering from serious mental health issues, it may also be a good idea to secure a note from a doctor or care provider to show your attorney, as there may be other issues of competency that will need to be addressed.
Each court's procedure is different. An experienced attorney can assist you with the procedures. Also, the attorney can file a motion to quash the warrant, explaining the circumstances and may be able to get a warrant quashed without your son having to go to court at all. If he goes to the clerk's office to quash the warrant, there is a chance he could be arrested there (depending on the court, the amount of the warrant, etc.)