J.B.
I interned with an investigative journalism news team in college and we did a story on pharmacy errors. The reported rate of errors for an average pharmacy is 2-4%. That means in a pharmacy that dispenses 100 prescriptions a day, 2-4 of them will be wrong. Every. Single. Day.
Luckily most errors are those where something like an antibiotic is dispensed at the wrong dose - say, 1.5 tsp three times a day instead of half a teaspoon three times a day. Luckily, most errors just result in minor discomfort (for antibiotics a high dose is hard on the tummy, exactly what a baby like your son doesn't need!) and not severe or fatal outcomes.
From that story, I learned to check every prescription. When I am at the doctor's office, I write down what they are prescribing, what it should look like, the dose and the frequency and I have them check my notes to make sure I got it right. I do this even when they do electronic presriptions. Then when I go to pick up, I verify the info on the bottle vs. what I wrote down. There was one instance several years ago where what the doc ordered wasn't covered by insurance so the pharmacist called the office and got a substitution, which they filled. It didn't match what I had written down, though, so I called the office to verify the switch. The nurse I spoke to said that they make these changes all the time and that I was literally the only parent who had ever called to question a change. That's scary.
I so hope that you little boy gets well quickly. Definitely make sure that the pharmacy reports the error (I think each state has a different oversight board) and in the future, check every prescription for an extra layer of oversight. We shouldn't have to do that, but it's the one step that we can take to help lessen the chances that our family will be the one getting the bad prescription that day.