Sons meds (amoxicillin) says to take two 20mg daily. he did this but at once. is this bad? or should he have waited an hour to take the other

Fine: Do not know what is being treated. Amoxicillin's half-life is such that it should be taken every 6-8 hrs at minimum for most infections. Every twelve hours would allow levels to fall below the inhibitory concentration for most organisms. Check with the doctor who prescribed this. Something may have been transcribed incorrectly.

Best to take 2x/day: Amoxicillin does not stay in the body for a long time so it is best to take the doses 12 hours apart to maintain steady blood levels. However, the instructions on the label were poorly written- it should say take one every 12 hours rather than 2 daily. It is not terrible if he did this once. But are you sure of the strength? 20 mg is a tiny dose, appropriate for someone who weighs about 5 pounds!