Have you even applied to a law school? Do you have any idea what questions are on the applications? They sound very serious when asking about any arrests and they are very serious about second-time applications. They want to know why you were rejected the first time. If he applies now and is rejected, he has to reveal the DUI arrest both cycles. If he waits until next year, then he very well might not have to even put DUI anywhere.
I disagree. The OP will not know definitively the reason for any rejections the first time around will be due to his arrest (although it would be nice if rejection letters included a specific reason or reasons for rejection!). It is my understanding that the apps simply ask if you applied before, the year, and the result (accepted, rejected, WL).
If the OP is not ultimately convicted of the DUI offense, he/she still needs to disclose the arrest and/or charge with the way this question is worded on many applications (a few apps come to mind - WUSTL, Duke). I also think even the schools that only ask for convictions also have a question about "are any charges currently pending against you," meaning if the OP applies this cycle he/she will have to disclose on every application. Being as this is so recent and unresolved, I am not sure what the adcomms would think... if the OP is an auto-admit applicant by the numbers, maybe this wouldn't be a big problem. But as far as being a borderline applicant.... I don't know.
However, if OP waits a year and ends up not receiving a conviction, there are many other school applications (i.e. Stanford, Berkeley) that only ask about prior convictions (and, I think, pending charges). The OP could then truthfully and accurately answer no on SOME applications.