If he doesn't contact DMV within ten days of his arrest to request a hearing, his drivers license will be suspended for one year. California has a "zero tolerance" law for underage drivers, and if a person under 21 is caught driving with a blood alcohol level of .01% or higher, there's a one year suspension.
If the juvenile court judge finds he committed the offense, that would also trigger a one year suspension (although his license won't be suspended twice for the same incident).
Public defenders ARE lawyers, but they are paid by the government to defend people who can't afford to hire an attorney. However, the public defender won't represent you nephew at the DMV hearing (and the deadline to request a hearing will be long past by his first court date, anyway).
Unless he has a substantial juvenile record, it's unlikely he will receive any punishment other than the license suspension.... although he could be placed on probation to make sure he stays out of trouble.
It may be well worth hiring an experienced DUI lawyer. Even a juvenile DUI will stay on his DMV record for ten years, and would substantially increase his insurance rates
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
The first question to resolve is whether or not the DMV hearing was requested within 10 days of the arrest or citation. If so, it would almost certainly be worth hiring an experienced DUI attorney.
Assuming that your nephew gave a breath test, the devices that most agencies use for a PAS test are governed by a different set of rules than the evidential breath tests that over 21 drivers are given. Many of the law enforcement agencies are not in compliance with these requirements when it comes to the Preliminary Alcohol Screening (PAS) devices, and an experienced attorney will know best as to how capitalize on these issues. Additonally, the same issues can arise at court in regards to the use of the PAS device. Good luck.