I too am a male in my 20s
Nice to meet you!
Being there for your brother will help. It may not always feel that way, but it does. As for T the end of my conversation with my T are about Dungeons and Dragons, Warhammer 40k, or school courses. The starts are also generally trivial. At times most of the session will be. This could be because I am in a good state and don't want to rock the boat too much that session, or because I am going to say or have said something really difficult. Therapy needs to be eased into.
The most important part of therapy is the relationship between therapist and client. Small talk can really be a tool for that.
Another important thing is that you do not -at all- have to understand everything to help. It's OK not to understand and just acknowledge that. You can still help by being there. Being there if he needs to talk about his shit AND being there if he wants to just hang out play video games or whatever it is you two can share.
Also let yourself acknowledge this is rightfully a hard situation for you. It is very hard to see someone you love suffer. Feel free to use this site for your own support as well. There is a friends and family section where others are coping with their loved ones "recovery". Also, feel free to message me if you are feeling you need to get something off your chest. I can listen, or offer my own two cents if you want that.
Sorry to hear your brother needs your support but it is really amazing that you are trying to help him like this. Thank you for being an awesome human being