How much trouble he's in will depend on his history up until now and how high his blood alcohol content was.
If he was over .16 BAC he is consider "super drunk". The court's experience with drunk drivers tells them that this is not a casual drinker that made a mistake, but a person who abuses alcohol and thinks they can still drive.
First offence cases that aren't "super drunk" can sometimes be plead down to "operating under the influence", this will not count toward future DUI's that trigger a felony on the third conviction. If he thinks he can ever have a drink and safely drive again he's in for a rude awakening. He should always lawyer up and try to lower the charge if possible. Sometimes a "reckless driving" charge looks better on a person's record than operating under the influence, some prosecutor's will consider this option. Job opportunities that require a driver's license vanish into thin air if a DUI is part of his record. Business owners are unwilling to assume increased liability and higher insurance costs associated with covering someone once they're convicted.
Many of Michigan's urban areas have "sobriety courts" which allows people to avoid jail time by volunteering to participate in an intensive probationary process. They have house arrest outside of court appointments and scheduled work times. They have a breathalizer installed at home at their expense $100 +/- per month and have scheduled times they must blow into the device which records their picture during the process.
Court officers, usually light duty officers or retirees under contract are tasked with arriving at the probationer's home unannounced to insure they are home and sober during curfew hours, usually around 2:00 a.m. to 4:00 a.m. If the probationer refuses to answer their phone or door at 2:30 a.m. when they show up it's weekend in jail minimum. Failure to comply with the stipulations of their probation results in weekend's in jail or revocation of the sobriety court option and 93 days in jail.
While this seems onerous, it allows that person to keep their job and home which would be lost in most circumstances. It forces approximately 1.5 years of strict sobriety instead of a jail sentence. Many of the people charged haven't been sober that long since they were in their early teens. It gives a person a running start at a sober lifestyle and insures that they aren't drinking at all during that 1.5 year house arrest. The people with advanced drug and alcohol addiction wash out or refuse to participate in the first place. It's amazing how many choose 93 days in jail rather than a strictly supervised 1.5 year probation.
Mandatory attendance at Alcoholics Anonymous (AA) meetings is part of their probation. Usually at least four to five meetings each week for the entire 1.5 year probation. This requirement must be met in spite of the fact that the probationer hasn't got a driver's license. Their friends, neighbors and families get stuck with the transportation of these men and women or the ride bicycles/walk to meet their obligations. If they're court papers don't match up with the other probationer's paperwork regarding signatures obtained at any given meeting they do a weekend in jail. If this happens again they can be kicked out of the program and have to serve their 93 days.
They also are required to attend substance abuse counseling at their own expense if they don't have insurance.
These court systems have taken extraordinary efforts to address the problem of drunk driving by affording the violators a real opportunity to look at and address their substance abuse. They also have no problem of sentencing them to 93 days if they should fail to tow the line. Your step son would be well advised to search out an AA group now, close to where he lives and start attending meetings and recording his attendance. The court likes to see people being proactive regarding the problem the court will think they have. The meetings won't hurt him one bit and he may even enjoy them.
Getting a DUI is the definition of a drinking problem. Having a hangover is a drinking problem. Spending more money than you wanted to last night is a drinking problem. Whether your son-in-law has the bigger problem I don't know. If he is an alcoholic this legal problem is just the beginning of his troubles. He is going to have a lot more trouble to come if he is. The worse thing you can do is give him financial support, it's important that he feels the full force of his decision making process. This may sound harsh, but it's sound policy. You won't feel betrayed when he continues down this rocky road. Your daughter's well being will become more important to you should he continue down the path of self-destruction.