Thank you for your query on Healthcare Magic.
The stomach pain may be due to gastritis
secondary to the infection or due to the drugs.
What antibiotic has been used and for how many days?
Are there any loose motions or vomitings?
Antacids like digeine or gelucil which are available on over the counter will relieve his pain.
If the pain persists consult your Pediatrician
for the examination and for specific treatment.
Hope I have answered your query.