cancel a tenancy at will at any time, but this kind of thing is pretty regulated by statute. Especially in a state of California, where the landlord-tenant laws are probably more pro-tenant.

To cancel the tenancy, you'll have to give him notice (probably 30 days notice). You can't use any "self-help" remedies like changing the locks or calling the sheriff. If you do, you could expose yourself to liability.

The best thing to do is to give him notice that you're going to sell the house and so you're terminating his tenancy in X days (you'll want to confirm how many days -- I assume it's 30 but it may be different). Then if he refuses to leave, you'll need to file an unlawful detainer or similar action in court.

You should probably hire an attorney from the get go to make sure you don't run afoul of any CA tenant laws.