Thanks for writing your query.
After reading your query, I would like to advise you that you can give your child of 12 years, ondansetron
You should also give him an antacid
15 mg along with the above mentioned medicine for better coverage.
Give him light meals prepared under hygienic conditions.
Maintain his hydration.
If the problem persists, get him examined by a medical professional.
I hope this is helpful to you.