Probably you will be getting into more trouble if you drink alcohol, even if it's "at work." The purpose of bond is to make sure you show up in court and don't get into more trouble while waiting for court. I'm sure you've noticed that some people who drink get into trouble, so the best way to avoid trouble until you're finished with your case is to avoid alcohol. You need an attorney to represent you on your current charge; don't make that attorney's job any harder; avoid alcohol.
You are asking the wrong question. You should be asking: how can I stay clean while on probation so I don't have to face further consequences? Rule #1 on probation is always "do not violate the law." Taking illegal drugs is illegal. Just don't do them and have a happy probation knowing you are doing anything to mess things up.
DUI laws vary from state to state. I'm sure that Washington lawyer knows the law in that state, but in Georgia it's different, and that answer doesn't apply to GA cases. I agree with Zachary Protor's answer. You need a lawyer if you expect to succeed. An experienced DUI defense attorney will know how to fight your case. It's not necessarily legal to drive when you're taking prescription medications. It depends on how those meds affect your ability to drive, and whether there are witnesses...
The question is not whether you are guilty, but whether the government has proof beyond a reasonable doubt that you are guilty. You are entitled to see the evidence against you, but you have to know the right way to request this. You need an attorney. These are felony charges.
I agree with George. Being in possession of less than an ounce is a misdemeanor, and while it carries a max of a year in jail and a $1,000 fine and a license suspension, more jail could be expected on the probation revocation. You need a lawyer.
Your son really needs to hire an attorney to either persuade the felony probation officer not to revoke probation, or if that process is started, to represent him in court. Unless he knows the rules of evidence, he cannot represent himself, and a non-lawyer family member can't do this for him.
He needs a TN lawyer right away. That TN lawyer should also consult with a GA lawyer to find out the consequences to your son's GA license (I'm assuming he has a GA license). Every state is different. In Georgia, it is nearly impossible for the prosecution to win a DUI-marijuana case. But a reckless driving will still cost your 19-year-old son's driver's license.