Thanks for your query,
As you said your son had severe chest infection
he must have taken antibiotic for it and diarrhea
is antibiotic associated.
I advise you to give probiotic
like lactobaccilus sachet for 3 to 4 days.,give oral rehydration solution sip by sip after each loose stools,give coconut water,curd,banana,boiled potato.stop pediasure for sometime.
In case symptoms persist take pediatric consultation.
Your child will be fine.