My question involves landlord-tenant law in the State of: California

I'm in a bit of a dilemma in terms of my roommate's security deposit. We are currently renters in the state of CA. Backstory is:

Roommate and I decided on a 12 month lease for a property. I signed the lease first since we were both out of state and I happened to be in the area at the time. He never signed it. We both split the damage deposit ($1500 each). Relationship became toxic in the house and roommate was trashing the apartment so I told roommate I wanted to move out at the 6 month mark (December). My roommate also agreed to move out with me. I informed my landlord that we needed to break out of our lease early (Nov) and she was accommodating but needed us to help show the property to new prospective tenants. I informed my roommate that we couldn't move out until we found new tenants (November) however he said "I don't know about you but I plan on moving out at the end of November whether or not we find new tenants." He ended up moving out on Nov 30th and I am still here paying rent and actively searching for new tenants.

My plan was to use his $1500 deposit to help pay for the rent in December since he left prior to us finding new tenants. Roommate thinks he is entitled to get his entire $1500 back. Am I obligated to return his deposit to him even though he never signed a lease? He sent me his share of the deposit electronically so I cut a check for the initial deposit of the lease.