I think you kinda answered your own question there, sounds like he won’t pay even if you did stop coming in. So the real question is would you want that project on your reel? If so and you have no other paying gigs set up, then I would get the experience. You also have every right to say that your internship is done and you’re going to work on personal projects, paid jobs, take a break, etc.
Businesses with poor managment practices like relying entirely on free labor although may make short term gains, in the long run will never make it with the big post houses. Learn and take what you can from a mediocre environment then move on.