There is a concept in the law called finality of judgments. Unfortunately, you are probably not going to be able to re-open the case. The reason is because I inferred from your answer that you plead guilty. The only exception will be if you have evidence that will prove that you are actually innocent. However, absent that, I am afraid that you are stuck with the situation. Your unfortunate situation is a perfect example of why it's import to not be wise to the penny and foolish to the pound...
You will not necessarily be placed in the "significant" risk category but your high BAC puts you at a disadvantage. It depends on the overall picture painted by your evaluation and to a large extent, the judge who has your case.
Many courts believe in work release as an alternative to traditional jail so as to give a repeat offender a chance to continue working while serving a period of incarceration. The key for you will be to be proactive on one hand but do not neglect to unturn every stone in analyzing your case. You should get the help you need from a skilled DUI attorney.
The short answer is: it depends. If the agreement was in Michigan and it was protected by special language placed on the record called Killebrew language then your recourse is to withdraw. If the agreement is protected by the Killebrew language - which your lawyer is to ensure that the record of the plea hearing contains this statement - you have the automatic right to withdraw. Otherwise, you have to tell the judge that you want to withdraw the agreement based on the interests of justice. I...
Your son should definitely obtain a lawyer. Foremost, the officer will be issued a subpoena by the prosecution. Secondly, it is quite possible that the officer was not forthcoming.
If you want to protect your son's record, help him find good counsel.
In Michigan there is a law that allows for diversion/dismissal of a charge of minor in possession/consumption of alcohol. There is legal authority here that prohibits officers from giving people breath tests to prove minor in possession without a warrant. Check with a lawyer in your state.
I need to know more to give you an educated answer. Generally speaking, searches of a vehicle following and incident to an arrest are invalid under Arizona v Gant. There are other exceptions such as an administrative search or a consensual search. However, the state has the burden to justify the seizure of evidence following a search without a warrant. I hope that helps.