How about just not tell him? Don't go to your appointments, don't go pick up your RX... His sure to see the difference in the billing, and you to can address it then. That way he has a chance to see you off meds (and hopefully better or atleast the same) without knowing it, and he won't have a leg to stand on when you raise that point?

Other than that? Idk sorry, I don't live with him, don't know him, so I can't predict his reaction. You know him best though... what would the typical response to "dad I flushed my meds down the toilet" be?