As I said and I assume that by now since your post was dated over 21 hours ago more in fact that you have already done so .would you let us know what the doctor said ?....
I agree with Caregiver. Please do not wait. Call Poison Control and talk to them. Chances are all will be well, but it is important to make sure.
The latest research on amoxicillian says that dosing should be done by weight, age being less important. However, it's important to not exceed the recommended dose by age, so this truly only applies to children under the maximum weight. And, as Caregiver said, the concentration is very important. Give Poison Control a call, that is your best resource.
on research I see that amoxicillin has a wide dosing range you should be okay, but of course get in touch with the doctor to make sure..
You mus IMMEDIATELY call the national United States Poison control number at 1-800-222-1222. This is a national number provided for the purpose of enabling anyone anywhere to immediately speak with a physician for advice when ingesting potentially hazardous substances. The oral medications come in various concentrations, so depending on the prescription you may or may not have a problem.
Was this his first dose of the day ? I dont think it will harm him but as a precaution call the Doctor;s office and ask them ...