First of all - that sounds incredibly hard and frustrating. It is great that you want to help your brother and help him deal with his illness. Bipolar disorder is a very hard and frustrating disorder to deal with for the both of you.
To start off with the positive - there are treatments that have been shown to empirically help bipolar patients. It is important to remember your brother is not his diagnosis - he is your brother and you love him. Do not reduce him to his diagnosis.
How can I get him to open up and talk to me, his younger brother?
There are a few things that you can do to start building this relationship.
Take care of yourself:
The first thing you might want to do about this is take care of yourself. Having an ill close family member is incredibly taxing on your mental health and the first thing you can do to help him and get him to talk to you is to make sure you're taking good care of yourself. This is "take care of yourself" as in:
- Consider talking to a therapist about all this this. Having someone on your side you can talk to in a therapeutic relationship has been shown to help. Having a mentally ill family member is very hard and it's important to acknowledge this and take care of yourself. If a therapist is out of question consider a spiritual leader. Support groups for people who have ill family members are also really helpful.
- Consider self-soothing exercises and emotion regulation. Mindfulness can be great for this (meditation) - there are other alternatives (prayer for example also helps).
- Make sure you sleep enough, eat healthy and get exercise. Make sure you get social interactions that are positive and build a support network.
Learn about his condition
Your brother sounds like a smart and capable individual dealing with a very challenging problem. You want to understand his perspective. You can do this by asking him about his life and then listening. This relates to the next point.
Be approachable and non-judgmental
This means that when you talk to him you hear him out and then acknowledge his feelings rather than attack him or his emotions.
Hey, you are my brother and I love you. I want you to know that no matter what happens I'm on your side and I'm always willing to hear you out non-judgmentally.
Repeat this as many times as needed. Tell him you believe in him. If he does talk to you about something do not judge him for it. For example if he tells you he has been sleeping on the roof - rather then tell him to not sleep on the roof or that it's "weird" just acknowledge that it must be tough and ask him why:
Brother: I've been sleeping on the roof in the last two weeks because my dream told me.
You: That sounds bad, you should stop listening to your dreams, you're smart - we had such high hopes for you why are you throwing things away?
That's bad because it's judgmental and it doesn't seek to understand.
You: Can you explain to me why you're sleeping on the roof?
You: Thanks for telling me. I value that you trust me enough to share with me. Do you think sleeping on the roof is making you happier? Can you explain to me why you're sleeping on the roof?
Encourage him to seek treatment by sharing positive stories about people who improved their life. This is something you should do after there is trust.
Acknowledge this is not something you can fix
This is incredibly hard and frustrating, but you cannot change people unless they want to and people with mental illness have a very hard time understanding they need help sometimes.
You need to be there for him and try to encourage him to get help but it is not your fault he is in this condition and it is not your responsibility.