You can consider yourself very lucky that you caught the mistake before your son took the wrong medication. Since there are no damages to your son, there really isn't much of a claim to pursue against Walgreens unless you could prove that the mistake was done deliberately.
You can inform the pharmacy manager of the mistake. You can also file a complaint with the agencies that regulates pharmacies and pharmacists in your state.
Everybody should check the labels before taking medicines. Many people do check. Some people do not, relying on the pharmacy to give the proper medicine. The people who do not check before taking the medicine sometimes end up dead for taking the wrong medicine.
In your particular case, since no one was physically hurt, you likely do not have much in terms of legal damages. So, besides filing the complaint in the hope of eliminating future problems, your only damage likely is a correct filling of the prescription. You can check with one or more attorneys in your state to see if your state's laws would give you more options.
You can file a complaint with the pharmacy licensing agency or licensing board. I am not sure you have legal remedies that are wroth pursuing unless your child suffers harm from the mistake, which you have not indicated whether or not he did.
On a side note, I think the pharmacy would be willing to work with you and give you a discount of some sort or something. A legal claim is probably not worth pursuing since you caught the mistake in time.
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