We have a 21 year old son who is on drugs and refuses to go to treatment even though we have offered to pay. The interventionist whom we hired has told us that we need to start the process to evict him if he continues to refuse treatment. We now need the legal method to do this eviction.

Give him a 60 day written notice to vacate. If he doesn't, then go to the local housing court and file for an eviction order.

You might get him under CA law for Lodgers, but if there is no rental agreement, I don't think that will fly.