Although you are not a mandated reporter under California Law, you may consider contacting Child Protective Services. At 17, the boy is considered a minor and the parents throwing him out is considered Neglect. They have a legal obligation to give him food and shelter.
Here is a list of the county Hotlines in California: http://www.childsworld.ca.gov/res/pdf/CPSEmergNumbers.pdf
This will help the boy get the resources he will need if he is unable to go home. CPS has access to shelters and emergency services in this type of situation. The boy will need health care, dental care and will have to go to school.
I don't know if you have any contact with the parents. I mention this to protect you from any allegations that the parents may make against you. For example, if they have asked you not to take him in, they can bring charges against you for Child Abduction. It is rare that the police would charge this, but you should be aware of the law. Here is what the law states:
CA Penal Code § 278 (through 2012 Leg Sess)
Every person, not having a right to custody, who maliciously takes, entices away, keeps, withholds, or conceals any child with the intent to detain or conceal that child from a lawful custodian shall be punished by imprisonment in a county jail not exceeding one year, a fine not exceeding one thousand dollars ($1,000), or both that fine and imprisonment, or by imprisonment pursuant to subdivision (h) of Section 1170 for two, three, or four years, a fine not exceeding ten thousand dollars ($10,000), or both that fine and imprisonment.
If you decide that this boy can stay for an extended period of time, you may want to consider legal guardianship through the Family Court. This will help you to manage educational needs and medical needs.
You may also consider calling or having the boy call the California Youth Crisis Line. This is a hotline that can put you in touch with services in your area. Here is the link: http://www.youthcrisisline.org/about.html
Please let me know if I can help more. If you are satisfied with my help, please rate my service.