Is he a signatory to the lease?
If he is, you and he are jointly and severally liable for all damages owing to the landlord.
That means he's a co-tenant.
That means your landlord is the Bailee for his property (Explained below).
If he's NOT on the lease, you're his Bailee (Explained below).
If he isn't on YOUR lease, he's potentially a guest in that he seems to come and go.
Either way, his property is the object of a bailment.
Bailment describes a legal relationship in common law where physical possession of personal property, or a chattel, is transferred from one person (the 'bailor') to another person (the 'bailee') who subsequently has possession of the property.
You could potentially be the bailee.
Notwithstanding, he being a tenant of the landlord, or your guest he's someone's bailor.
You or your landlord are the bailee.
He's the bailor.
What are you supposed to do with your roommate's abandoned property under California law?
Unfortunately, as with most legal issues (and yes, this is a legal issue), there is no simple answer. Your proper course of action depends upon the specific facts involved, as cited hereinabove.
What is determinative is if you are considered his landlord when the person abandoned the unit and his property.
In California, a landlord MUST provide written notice to a tenant that has left property in the unit.
The landlord is also required to give the former tenant a grace period of about 15 days to pick up the property.
If the landlord fails to follow these requirements, the landlord could be liable for the loss.
In the case of a roommate, you may or may not be considered your former roommate's landlord. If you are not the landlord, it may be best to contact your landlord and have the property owner deal with the situation.
If you are the landlord, you may want to look into the specific rules and procedures for returning abandoned property in your California.
If you're unsure about your responsibilities, or if the former roommate/former guest might sue you over the abandoned property, you may want to consult with an experienced landlord-tenant attorney to figure out your best move.
Read these links:
http://www.dca.ca.gov/publications/landlordbook/catenant.pdf
California Tenants - California Department of Consumer Affairs
How to Get Back Possessions You Have Left in a Rental Unit: Legal Guide LT-4 - California Department of Consumer Affairs