No one can tell you his odds over Avvo. There are too many variables, and he MUST meet with a criminal defense attorney in confidence to reveal his while story. If he cannot afford one, it is his right to have one appointed free of charge.
Honestly, the best thing you can do for him is to help him find an attorney immediately. Don't put any more details up online. It can only hurt his chances, whatever they are. Anyone can read Avvo. Cops, prosecutors, etc.
In the mean time, he should talk to no one about his case, including you, except his attorney. (You and he are not yet married, I take it, so his words to you are not yet protected by law. Only his words to an attorney are.)
If anyone does come around asking questions, he should say only the following: "i invoke my right to remain silent. I will not answer any questions without an attorney present, and I want my lawyer now. I do not consent to any searches of my person, property or home."
Best of luck!
Dear Asker: This answer does not constitute legal advice, and I am not your attorney. No attorney-client relationship is established between me and you by my answering your question. You can receive general information about the law here on Avvo to help you understand it better, but if you want actual legal advice, call an attorney for a private, confidential consultation.
I just wanted to add to what Ms. Mockrin said: felony DA's don't mess around. They will record every jail call and play any bad ones at trial; I would not put it past them to do an Avvo search to see if anyone asked for legal help for him online. In other words, do not post any more details of his case on the web. He should sit down in an office conference with a DUII specialist. Since you are not married, the DUII lawyer may ask you to leave the room. The attorney-client privilege includes parents and spouses explicitly -- but not people who are engaged. Also, married people have separate rights of confidentiality in both the marital communications and spousal privilege not to testify. This is just a quick heads up that you shouldn't be offended if you do all the work, find a great DUII lawyer, and then the lawyer excludes you from the room whenever your fiance is speaking to the lawyer. On the other hand, there's nothing wrong with you being in the room for general advice that doesn't reveal trial strategy, etc. I'm sorry your family is going through this, and I wish you all the best.