Mania as severe as you are describing is definitely something to worry about. You're not likely to convince him while he's manic to go back on the medication that will bring him out of this manic (and really fun for him probably) stage. That being said, I would certainly continue to try to reason with him and definitely inform his doctor and therapist if he has one.

If you fear he is a danger to his life or the lives of those around him, even through negligence, I would take it up with the proper authorities immediately. I don't know where you live but my city has a nurse hotline available free 24/7 and they would be the people I would call to help me sort out what is likely to be immanently dangerous and what's not. It's a scary slippery slope and I wish you the best of luck. I also feel a strange sense that perhaps you're searching for permission to step in and help? When I was at my worst I wish someone had taken action for me or even helped me to take action. I ended up calling 911 myself and voluntarily committing myself but that is not the case with all patients with bipolar disorder. Please do what you need to to keep everyone safe