Most Helpful Guy
Let me say this as absolutely gently as possible: At 15 years old, there is nothing you could possibly do.
At 15 you just do not have tue emotional intelligence or fortitude to handle the mental ans emotional strain of being in a relationship with someone who is unhealthy. It will wear you down and eventually hurt you or push you out of the relationship. For his sake I absolutely hope he is seeing a professional who has diagnosed this matter and he is receiving medication and counciling. None of that involves you, however. If you choose to input yourself at this age the ONLY thing you will do is hurt him. Because you will be distracting him at an age where he is already a hormonal mess and then throwing in mental health issues. That will lead to a potential worsening of his situation and then eventually when you guys don't work out because he either chases you out his his problems or you leave because it simply doesn't work out; the damage to him will be far more severe when compounded by a spike in depression and inability to reign in his negative cycles.
Not to be mean. There is just nothing you have to offer this situation that is beneficial. If you date him, keep it lught, fun and casual. Don't get sucked in. Don't try and be his hero. Don't be a crutch. Don't be an enabler. And since that is a minefield, that again at 15 you just don't have the wisdom to trek without hurt, I would highly recommend not being involved at all.
Most Helpful Girl
You should do a lot of research on bipolar disorder - understand what it means, the symptoms and forsee how it could impact your relationship so your one step ahead in understanding his bevhaviour and thought patterns in certain situations.
Understand that you can only support him and control your emotions and behaviour. I always see people trying to "help" someone with a mental disorder but its in their power to help themselves; not you.
Also ask him how he would like to be supported, be empathetic and most importantly... treat him like everyone else! His not disabled! 😊