hi... you child has got gastrointestinal infection
which is leading to green coloured stools.. you said reducing substance in the stools is due to lactose.. there is a transient(short period) lactose intolerence due to the infection... there is a shedding of mucosal lining with diarhhea causes loss of lactase enzymes.. hence lacotse in the milk is not digested in the gut and leads to acidic stools..
see your pediatrician
for antibiotics, correction of dehydration
, probiotics and clean water and advice on diet.. better to follow lactose free diet.. all the very best..