How could I evict my step son?

He took possession of the property with your permission. He is either a "tenant-at-will" or a "month-to-month tenant," depending on whether there was an agreement between you and him that he would pay rent (whether or not he has actually paid rent). You must serve him with the correct notice, wait the required amount of time, and then, if he has not voluntarily vacated, you must go to court to evict him. Because the type of tenancy is not clear, you should hire an attorney to handle this eviction for you.