Good heavens. No, you don't get a lawyer. You're focusing on the wrong thing.
As others have said, you teach the child not to hang out with kids who pull "pranks" like that because it's stealing. (Drinking something without paying is stealing.) It's also disgusting - putting a bottle back on the shelf where perhaps some little kid could find it, now that the childproof seal is broken. Let your son know that choices he makes have repercussions, and that he needs to have exemplary behavior going forward.
If the manager knows your son didn't do it but the employee is telling people he did, then you let the manager know you won't be patronizing the store anymore since it's too risky to be in the company of this employee. You do this quietly, without emotion, without threats - just a simple statement. Maybe the manager will straighten out the employee, maybe not, but that's not your call. You control what you can - your own actions and your supervision of your child.
If you make a huge deal about this, you will reinforce people's idea of your son's guilt. Stop. Take the high road. Say, "No, that's an error. As soon as I heard about this, I took my son right down there, and the manager has confirmed it. I have no idea why some people would spread this rumor." And that's all you say. The more you rant, the worse it looks for you. And you can't use terms like "drunk some Gatorade" or "questing us" because it doesn't make you look particularly reliable. Let this go.
ETA: After reading your So What Happened...You have just got to get past the idea of "good family" being a barrier to bad behavior. Define "good family" please. Sometimes kids go astray, but sometimes people look at whether a family is employed, active volunteers in the community, personally charming, etc. and they act like that's insurance against kids being thieves or drug addicts or sexual deviants. It's just not. You can't "tell by looking" whether someone is stealing or beating their wife or anything else. Everyone is capable, my dear. The definition of a "good family" is one that takes this boy's actions seriously and imposes consequences. As should you. Get past the apology - stop waiting passively for it. Take your son in to the manager, have your son tell the manager he won't associate with that boy unless and until the boy makes restitution AND does some volunteer work, and show that you are deserving of the honorable treatment you want. If you hold your head a lot higher and stop with the righteous indignation, you'll defeat the rumors a lot more effectively than if you keep going with this "small town/everyone's talking" routine.