Is he paying rent? if not, then pack his bags and leave them outside the door, change the key and go to your mother's for a week or two.
If he is paying rent, then you have a Tenant and you will need to go thru the eviction process.
Simply stated, it's time to start advocating for yourself by asking them to make arrangements to move on. Setting a date for departure can be helpful....or start charging them rent....a big rent!
If you feel that you need some support, have an intervention, with some better friends by your side.
B) Make sure you fail to do your laundry until you can't see the floor.
C) Buy a pet Possum.
D) Get a few more "roommates" from your HS or Grammar-School (blood-brothers) with strong defined written agreements, time-limited until "friend1" clears out. No reason to let the new "roomies" into your space; i couch-surfed for a decade, i was so nice (NEVER leave a toothbrush) that i had 25 door keys as that "homeless" guy:)
CAUTION California law is nasty, your "sub-tenet lease" is automatically created EVEN with $0 monies involved.
You must have a real locking safe (for them to keep their valuables) for a day-to-day or week-to-week arrangement. And, even if he leaves, you must retain his belongings for 14 days and NOT profit from their disposal.
Frankly, peer-pressure is the only safe route, and hisStory is usually BS, but will often trump yourStory. See A-D.
Hope this helps!