I do not practice in MI, but I may offer some general advice. You son needs to turn himself in on the bench warrant, but he MUST speak with and (if possible) retain an attorney before doing so.
Your son's situation is very sympathetic, and a court may be inclined to give your son more time to complete the terms of his sentence. But given his disability, he will need assistance presenting his case to the court, and his chances of being allowed more time increase exponentially if he has an attorney on his side. A local, experienced attorney will be familar with the process, the judge, the prosecutor, etc. and use that familiarity to your son's advantage.
If your son cannot afford an attorney, he should contact the public defender's office for the jurisdiciton in which his case is pending and see if they can assist him. If your son does not qualify for public defender assistance, you may want to try local bar associations and law schools for pro bono or low-cost assistance.
Consult with local, experienced DUI attorneys in MI, preferably ones with appellate or post-conviction experience. Interesting questions to pursue would be whether your son's plea was knowingly, intelligently, and voluntarily made; and whether he was denied his 6th Amendment right to counsel. Under the afore-mentioned grounds, he may be able to withdraw his guilty plea (Although, that would likely be an uphill battle; and it would expose him to defending the case all over again.). These questions, however, are for an appellate or post-conviction lawyer. Most initial consultations are free. So, you have nothing to lose at first.
Finally, you may want to try posting this in the MI forum, as your son's case is in MI. You may receive more specific answers from DUI attorneys in MI.