Any time a person violates the terms and conditions of their probation, they can go to prison for the full term that can be imposed less any time they have alresdy served.
The above is not intended as legal advice.
Yes, he faces a maximum sentence of three years, minus the time that he already did, but it is unlikely that he will go to prison at all for this. Most likely, he will just get reinstated on probation with a little bit of jail time tacked on for failing to report. However, if there are any circumstances that you haven't mentioned that make his situation worse, he could get a more harsh sentence. The maximum is only very rarely imposed.
The punishment he faces is discretionary with the Judge, based on what he pled to while he was in court when he first took his plea. You should be calling the attorney that handled the case for the plea.
While some may think this situation is not a big deal, I beg to differ. I actually practice crimial law in Kings County. The Court and District Attorney's Office is proud of the fact that last year they sent more people to prison than the rest of the 54 counties in California. They don't mess around in Kings County. Get an attorney involved as soon as possible.
To get a real answer, you have to ask an attorney who knows that exact court and how it operates. Some courts have a standard punishment for 1st time violators, on certain offenses. Often a first time probation violator gets more local time with probation reinstated, but in any given case the court can run out of patience and send a fellow to prison. You mention "enhancements", which in a 23153(a) could mean "great bodily injury" enhancments. Whether the cousins agreed or not, that is water under the bridge. IF there are GBI enhancements, the max is greater than 3 years, and any prison sentence is served at 85%. You mention that "it was his first and only DUI", but if his record is otherwise bad (e.g. other criminal history, series of other probation violations), the results could be different.