I don't know the legal procedures, but doing a Google search with "Eviction California" and "Evict a roommate California" brought up a lot of useful sites.
This site about evicting a roommate seemed particularly useful:
Even though the roommate is not on the lease, from what I read on several sites, it looks as though he would be classed as a tenant, because he has lived with your son for 30 days or more. Did your son put the 30-day notice in writing? The various sites seemed to indicate that this might be necessary and several offered suitable forms.
"Illegal activity" was one of the grounds given for legal eviction in CA. If the chap bought stuff online using your son's card, could he perhaps class that as illegal activity on the premises and use it a basis for the eviction?
Incidentally, more than one site stated categorically that a landlord was not entitled to get rid of a tenant's property in the interest of eviction.
Did your son's tenant put down any kind of deposit? My son is not in California, However, he once had a housemate, on a trial period, who was not a criminal, but who nonetheless turned out to be completely unsuitable. (I won't go into the gory details!) The young man was not working and was thus short of money. So my son offered to give him back the last month's rent, normally covered by the deposit, if he left early. The housemate accepted with alacrity and left almost immediately. My son was out-of-pocket, but considered his temporarily straitened financial situation to be well worth it, in order to be rid of the guy as soon as possible. (Obviously, he made him sign for the money, to cover himself.)
[This message edited by Cally60 at 1:05 PM, November 25th (Monday)]