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I'm sorry to hear about your situation. Unless he acted in this role as an attorney, the fact that he is an attorney and did this is irrelevant. The state bar is concerned about how he practices law and holds himself out as an attorney. Breaching an agreement, particularly where he is a party and not the attorney, is not going to be something that the state bar is going to be interested in.
At best you would have a breach of contract case against him as you had a signed, written agreement between you three that he breached. If you can prove actual damages that resulted from that (that you lost money that you otherwise would have had, or that you failed to make money that you could have made but-for his actions) then you could have a case against him. Otherwise, it's going to be something that will need to take its course.
Now you certainly can report him to the California Bar. But they wouldn't do anything, as he was not acting in the scope of actually being an attorney. The fact that he breached the agreement AND that he's an attorney are two unrelated things. He could have breached if he wasn't an attorney. So the state bar is not interested in regulating that behavior.
If you CAN prove damages, you would want to talk with an attorney in your area that deals with breach of contract cases. The attorney would be able to tell you how courts are treating cases such as yours in the area, assess the strengths and weaknesses of the case, and tell you what you could do next.