You can try calling the police department and ask if there is a crisis center where you can take your brother for immediate evaluation. It really depends on where you live, but I would try the police first, you can also look on the nami website and see if they have anything that might be helpful to you, a number to call. Good luck, I've been in the same position of trying to get someone immediate help and unfortunately it is very difficult. If all else fails, honestly I would take him to his psychiatrist, or psychologists office and sit there until he/she will see him and let them know its an emergency. Drastic times- drastic measures. Hopefully he is cooperative. BUT if he isnt, as far as you talking to his therapist- you are allowed to tell his therapist what is going on, but she is not allowed (by law) to give you any info regarding his case/ treatment, but you telling her what is going on is just as good.
Your brother needs to talk to his therapist and stress the severity of this episode. There are many reasons why the therapist may have missed or failed to return his call. IT IS NOT PERSONAL. Your brother needs to be proactive as a patient, and you need to be proactive as his support system. If your brother's therapist continues to be unresponsive though, you need a new one.
I think Cognitive Behavioral Therapy may be helpful for your brother. This is the standard treatment for people with anxiety/ bipolar and helps patients develop the tools they need to deal with their condition.