Regardless of the fact that he confessed his guilt to the officers, he has the same rights as any other defendant when he walks into criminal court. By that I mean that every person charged with a crime in the US is presumed innocent unless/until proved guilty beyond a reasonable doubt.
That gives your son the Constitutional right to plead not guilty at his arraignment, and that's just what he needs to do. It is the only plea that keeps all of his rights open to him, so that he can retain and get advised by counsel, have his lawyer bargain with the prosecutor for a favorable plea agreement, or fight the case. Any plea other than not guilty will result in an immediate conviction.
Given his admissions, he's unlikely to want to try this case, but he should never plead guilty unless/until he knows what the sentence is going to be in trade for his guilty plea. Once he has a lawyer -- and it's best that he gets one -- his lawyer can work on a deal for him. THen, if he gets a deal he likes, he can take back his plea of not guilty and enter a plea of guilty to get the promised deal.
You can always change your mind and go from not guilty to guilty, but it doesn't work the other way around. Once you plead guilty, you can almost never get back your guilty plea and become not guilty again.
If he has no prior DUI cases, he can generally avoid jail, but he'll be expected to install an interlock in his vehicle, and he'll face a fine of at least $250 and a license suspension of up to a year. The prior domestic offense would not enhance his penalty here for this kind of an offense.
To keep his fines and suspension to a minimum, he should have a lawyer representing him. If he's unable to afford one, he can plead not guilty at his arraignment and then ask the court to appoint him a public defender.