Thank you for your query on Healthcare Magic.
Your child is having diarrhea
mostly due to a virus but it may be due to an allergic because of millet porridge as it developed after starting the porridge.
Stop the porridge for now for some days.
Give sips of oral rehydration solutions like Pedialyte
to maintain hydration.
Monitor hydration by his activity, urine output, and skin turgor.
Consult the Pediatrician
for the physical examination of the child and for treatment of diarrhea.
Hope I have answered your query.