What is the recommended dosage of amoxicillin for a 14 year old child?

The recommended dose of amoxicillin is:

For oral dosage forms (capsules, oral suspension, tablets, and chewable tablets):
Adults, teenagers, and children weighing more than 40 kilograms (kg) (88 pounds) — 250milligrams (mg) to 500 mg every eight hours or 500 mg to 875 mg every twelve hours, depending on the type and severity of the infection.

However it is important to follow your prescribing directions.

For more information see:
https://www.drugs.com/cons/amoxicillin.html