How do I file for temporary custody of my son ASAP? 

If you can't afford an attorney, The Family Law Facilitator's Office in the courthouse in your county will be able to help you. This Agency is usually headed up by an attorney who is a Certified Family Law Specialist. However, I highly recommend that you hire your own Certified Family Law Specialist to handle the ex parte application and hearing for you. You'll find such a list at www.calbar.org under Certified Specialists. A change over of custocy is seldom granted on an emergency basis unless you can show that your son will suffer irreparable harm if the court does not grant the temporary order. That's why you need competent counsel.

Generally, if you need to file for a Temporary Emergency Order of Custody because the circumstances warrant such and you can not wait for a formal custody hearing.  The Family Court in your area allows for such a Petition to be heard.  Family Court venues often allow you to do this without the use of an attorney.  However, I would seek legal help with this matter as soon as you can. You will want to file a formal custody petition on the issue.  Good luck.