This response is given for informational purposes only.
If your son was on probation when he was arrested for the DUI, it was not his first offense, though it may well have been his first DUI, which perhaps is what you meant.
He now faces two separate prosecutions: one for the violation of probation, wherever he was serving that probation, and one for the DUI. Since he is not under sentence for the DUI, but is instead just accused of it, I am going to assume you are asking if he can receive a modification for the sentence for which he is on probation.
There is definitely a mechanism where you can move for a sentence modification, but it has to be filed within the term of court during which your son was sentenced. Assuming for the moment that your son was not placed on probation very recently, a modification probably will not be available. Terms of court vary among the various judicial circuits in Georgia and it is best to speak with an attorney to find out exactly when your son's opportunity to modify his sentence/probation expires or expired. However, if that time has already passed, the court which sentenced him is powerless to modify the sentence.
Your son may be able to attack the sentence which resulted in his probation collaterally, but that will take time and, probably, money to hire an attorney familiar with post-conviction work to review his case and possibly file a petitition.
If he is waiting to enter a treatment facility for six months and has already been locked up for about five months, his attorney may be able to negotiate some kind of concurrent punishment for the latest charges and his probation, but that will depend entirely on a number of factors which we don't know from what you have said, such as your son's criminal history, the charge for which he was on probation, the state's position on the matter, the judge's position on the matter, etc..
Obviously what I'm leading you toward is to consult an attorney about your options at this point.
He can try to send a letter to the court asking that his sentence be modified. He could also try to get his attorney to assist him by filing a motion to modify his sentence. However, it sounds at this point like his case has been resolved and he is being sentenced to in-house treatment and being held in jail until a bed opens up. The problem is going to be is if he had a court-appointed lawyer most of us, depending on the terms of our contracts with the court, usually consider our professional obligation concluded at the time of sentencing. I don't think that these efforts are going to be very successful though. While it is unfortunate as to what is going on in you and your husband's life obviously your son has put himself in a position of not being able to provide assistance, and in my experience this is usually how the court views these matters.