What is the correct dosing of zithromax for my 12 year old son who weighs 113 pounds?

Depends. If pills: two 250-mg tablets the first day, then one 250-mg tablet on days 2 - 5 (until med is gone) if liquid: 12.5 ml (use measured dropper) once a day for 5 days.
Whyis it given? Doses may vary depending on the condition that is being treated. The standard dode may not apply to certain infections. If you are doubtful, call the physician that has prescribed it for your child and voice your concerns.