California law doesn't allow for you to just change the locks. Follow procedure for an unlawful detainer as though he had a written contract with you. Since he has not paid rent, you can serve him a 3 day notice to pay or quit. It sounds like this tenant might believe he's in the right, so I suggest getting an attorney on your side in case this renter contests. Additionally, notices to pay or quit must follow specific legal requirements or they will not be upheld in court. Finally, it sounds like you may also be able to file a breach of contract claim against the original lessee. Depending on your contract terms, original lessee may still be liable to you for the rent of lessee's subtenant.
This reply is provided for information purposes only and does not represent legal advice or an attorney-client relationship.