No. Under California law, there is no obligation to continue paying child support for your child given the facts you have stated and there is no obligation to pay for college. Absent an agreement between the parties that is then made into a court order, the Court has no authority to order a parent to pay half of reasonable college expenses.
This information is provided for general educational purposes only including answers posted to questions asked. It is not intended to be relied on as legal advice. Your particular facts and circumstances must be considered to determine appropriate legal advice. Always consult with a competent attorney, licensed in your state, to discuss your particular situation. The material contained in this answer is provided solely for informational purposes. This information is not intended to create, and receipt of it does not constitute, an attorney-client relationship between you and the responding Attorney. The information in this answer is not guaranteed to be correct, complete or up-to-date. It should not be relied upon or construed as legal advice. You should not act or elect not to act based upon this information without seeking professional counsel.
Unfortunately, not in California. As far as California law is concerned, your son's an adult; any arrangement for the payment of his support or college expenses by Dad is going to have to be by agreement, either between you and Dad, or between son and dad.
No. Unfortunately in California, once your child turns 18 the court loses jurisdiction, and college expenses are not something the court will consider in continuing child support past the age of 18.