If you are not still using miralax and if he is still having diarrhea, it could be an infectious or non infectious diarrhea. keeping in mind the duration of the diarrhea, it looks like it is non-infectious. can you please tell me more that? like if he had any fever, chills, abdominal cramping, any other person with similar complaints?

Gas, Flatulence, Cramping along with Green Stools and 3 weeks duration, all points to a non-infectious cause. Also lack of fever. Importantly we need to hydrate your kid, so as not to dehydrate him.
Non-infectious causes of diarrhea can be of so many types including mal-absorption (inability to absorb) of some things, some autoimmune conditions, sometimes even dietary changes. If the stool sticks to the commode, it could be a fat malabsorbtion. It is hard to say as it has a long list of differential to be sorted out.

I would suggest avoid gluten containning food like bread or any wheat containing food. Also some people cannot tolarate milk products. Try to avoid that too. Though there are specific tests for each type of intolerance which you can get tested after visiting doctor, I think we can start with this trial and error if you are concerned.

I hope this helps.