You are not mentioned whether your child have diarrhea
with green stool
or only passing green stool.
If your baby have the occasional green stool - by all means don't panic if it is not related with Diarrhea!.
Think about the foods that you gave to your child ,
or supplements .
Yes your child have low weight as expected age.
For that you have to take care with proper and regular diet.
Child does not accept new food easily. Try one type of food for several days.
if possible. you can get back to me with the progress in case u need further suggestions.