Thanks for your query.
Since you have mentioned that your baby is exclusively breastfed.
In is hardly any possibility of amoebic infection in 5 month old baby who is having only breast milk.
I suggest you to get a second opinion
and get stools examined by second lab.
sachet for 2 to 3 days.
Give oral rehydration solution sip by sip after each loose stool.
Monitor for features of dehydration
like lethargy/decreased urine output/refusal of feed and in case you observe any get him examined by a pediatrician
Wishing your baby good health.