Sorry to note that you feel helpless with your current situation.
in a three year old can have several reasons:
1. Depending on the geographical location, coeliac disease (immne reaction against wheat proteins) definitely must be excluded. Blood tests such as IgA-TTG and IgA-endomysium
are available, and duodenal biopsy is the final test if serological tests are inconclusive. I am not sure why a diagnosis of lactose intolerance
was made with the biopsy as it is NOT possible to say, this needs to be discussed with your doctor.
2. Lactose intolerance is far commoner than coeliac disease, and complete avoidance usually leads to normalisation of GI system, unless he has a concominant wheat intolerance as well.
3. Irritable bowel syndrome
can happen in kids, although toddler's diarrhoea at this age is also a possibility that settles on its own as he grows up.
4. Infection-related diarrhoea needs to be excluded, or whether he is antibody deficient (such as IgA deficiency
, or hypogammaglobulianemic [all 3 Ig isotypes low] which can be diagnosed with a simple blood test.
I hope that was helpful.