Thank you foe your question.
From the symptoms of your son,it seems to Bacterial gastroenteritis
However to confirm it I need to know certain answers.
What is the age of your child?
Had such illness present in past?
What antibiotic you gave?
Did the child had worm infestation?
Is it associated with vomiting
,passage of blood in stool
Is the child weak and dehydrated?
I will advice you to have some investigations done:
Complete Blood Count
Stool culture and sensitivity.
Child must be well hydrated.So give oral Dehydration
Ensure that the child is on proper diet.
Start antibiotics only after the stool culture report comes.
Wish you and your son healthy life.